The Gloucester subplot mirrors the main plot of the drama. In the main plot, Lear is betrayed by his two elder daughters because he believes their flattering words and gives them his power. He thinks they will continue to honor and care for him. Likewise, Gloucester is duped by his illegitimate son Edmund into turning on his legitimate son Edmund—and ends up similarly betrayed by one he believed he could trust.

Both men thought they were wise in the ways of the world, and both, humbled, find themselves cast out into harsh nature to face themselves and the world without the trappings of power and position.

The subplot fulfills several functions. First, it shows that the betrayal Lear experiences is not unique: the young will betray the old, and those with power must be careful not to be too trusting. The desire for power is a huge temptation. Edmund, Goneril, and Regan all believe they deserve the power they have grabbed and that they are justified in using it immorally to achieve their ends. All three become more corrupt as they gain more power, until, for example, Albany rejects Goneril as nothing more than a fiend or monster.

In contrast, suffering and loss of power brings characters like Lear and Gloucester to greater wisdom and insight. Because these two men are undergoing the same experience, they are able to talk and commiserate with each other and with Edgar. In the end, these two once great men accept the reality of their common humanity; like everyone else they are, as Lear puts it, looking at Edgar:

no more but such a poor, bare, forked animal as thou art.

Having more than one person going through the same experience gives Shakespeare scope to explore it more fully and with more drama.

Gloucester acts as a foil to Lear. This means that he provides a mirror to the main character, whose traits are then highlighted for dramatic purposes. In structural terms, there are great similarities between the two men and their respective fates. Both come to grief through a naive trust in their children; both suffer appalling pain: mentally in the case of Lear, physically and mentally in the case of Gloucester; both realize, all too late, which family member was the one who really loved and cared for them.

Yet there are important differences between plot and subplot. Gloucester's suffering is more recognizably human than the mental anguish endured by Lear. Lear remains a larger-than-life character throughout the play; he may have renounced his kingdom, but he still acts and feels like a king. His lapse into insanity stems largely from his inability to come to terms with his humiliating status as a king without a throne.

Gloucester is more human and less exciting as a character, perhaps, but none the worse for that. And unlike Lear, he gives—not out of an insecure need to be loved, but out of a genuine sense of humanity. Both men ultimately fall, but for different reasons: Lear cannot live in a world in which he has no place because he is too God-like; Gloucester, a mere mortal, cannot live there either, but because he's all too human. The Gloucester subplot reminds us that what happens to Lear can happen to any of us, albeit for different reasons. 

In King Lear, the Earl of Gloucester’s storyline resembles Lear’s. In both cases, a powerful older man thinks he knows his children better than he does. They turn against their loyal children and are duped by their duplicitous children. Lear disowns the honest Cordelia and gives his kingdom to the flattering Goneril and Regan. Gloucester’s illegitimate son Edmund tricks him into believing his legitimate son Edgar is plotting against him. Goneril and Regan strip Lear of power, shut him out in the cold, and defend their actions in a war that leads to Lear’s death. Due to Edmund’s scheming, Gloucester loses his eyes, and his heart eventually gives out.

There is redemption in both of their stories, however. Lear reunites with Cordelia before they die, realizing that he is “a very foolish fond old man.” Edgar also meets Gloucester, cures him of his desire to kill himself, and eventually reveals himself to him. This revelation gives Gloucester joy but also kills him: his heart, “'Twixt two extremes of passion, joy and grief, / Burst smilingly.” One of the lessons learned in both plots is that a man, especially a powerful one, can be too comfortable in his ignorance. Gloucester is less ornery than Lear, but he is careless about Edmund’s mental state. Both fathers are humbled to discover how deceiving appearances can be, even in their own families.

What is the purpose of the subplot in King Lear?

The subplot in King Lear involves the downfall of the Earl of Gloucester, who is betrayed by his illegitimate son, Edmund. It provides an unusually close echo of the main plot, with Gloucester taking the position of Lear and Edmund paralleling that of his treacherous daughters, Goneril and Regan. Edgar, meanwhile, stands in the place of both Cordelia and the Fool.

Subplots often provide variety and relief, but this subplot has the opposite effect, reinforcing the atmosphere of terror and treachery with a villain more Machiavellian than Lear's "pelican daughters" and an event more horrible than any other in the play, the blinding of Gloucester. This subplot, therefore, intensifies the tragedy of the play rather than relieving it.

The tragedy of Gloucester also lends a universality to the tragedy of Lear. Although it is only one more example of conflict between fathers and children, the subplot suggests that the issues King Lear faces also occur among his subjects. Failure to understand where true merit lies and treachery within families are not solely the prerogative of kings. The subplot also intersects unusually closely with the main plot and gives the stricken Lear a companion in misfortune, as well as providing a manipulative and sinister partner in crime for both Goneril and Regan.

What is the relationship between the main plot and subplot in King Lear?

Shakespeare's King Lear is not the story of a single man who is betrayed by his daughters but a story about the universal tragedy of old age--how one generation is inexorably replaced by the generation it created through love or lust and nourished until adulthood. That is why there are two plots. Lear has daughters, Gloucester has sons. Both men find themselves out in the cold, stripped of everything they used to own, including their titles. But this is a universal theme. It has been going on among us Homo sapiens for something like seven thousand generations. Shakespeare shows this happening to aristocrats because that was the tradition in drama--but it happens to everybody. Parents typically love their children, but children do not necessarily care about their parents after they themselves are grown up and have developed adult interests in survival and procreation. In Measure for Measure, the Duke disguised as a friar tells the condemned prisoner Claudio:

Friend hast thou none;
For thine own bowels, which do call thee sire,
The mere effusion of thy proper loins,
Do curse the gout, serpigo, and the rheum,
For ending thee no sooner.      III.1

At least that was Shakespeare's view. Your own children can hardly wait to get rid of you and to get their hands on your property. This was certainly the attitude of Goneril and Regan with regard to their father Lear and the attitude of Edmund towards Gloucester. 

Old age is also a time of regrets. This is symbolized by Lear's bitter regret that he disowned the one daughter who loved him in favor of the two who cared nothing about him. When he meets Cordelia again near Dover he tells her in a stunning metaphor:

You do me wrong to take me out o' the grave:
Thou art a soul in bliss; but I am bound
Upon a wheel of fire, that mine own tears
Do scald like molten lead.      IV.7

This is a good description of the sufferings that go with old age.

What makes old age hard to bear is not the failing of one’s faculties, mental and physical, but the burden of one’s memories.
--Somerset Maugham

Gloucester feels exactly the same way about Edgar as Lear feels about Cordelia.

O dear son Edgar,
The food of thy abused father's wrath!
Might I but live to see thee in my touch,
I'ld say I had eyes again!     IV,1      

Both men's bitter regrets actually symbolize the regrets that all old people feel in old age. Gloucester disowned Edgar, the son who truly loves him, in favor of Edmund, the bastard son who cares nothing about him but only wants his lands and title. 

The two old men are accidentally brought together in a pathetic scene in Act IV, Scene 6. They are hungry and dirty. They have been stripped of everything and are living on weeds and mice. Gloucester has even lost his sight. This great scene symbolizes the fate of every generation when it is no longer wanted and is only in the way. In giving Lear daughters and Gloucester sons, Shakespeare intended to represent the whole human race, each generation treading on the heels of the generation that came before it. The characters who love these old men--Cordelia, Edgar, and Kent--exist primarily to serve as contrasts to the ones who care nothing about them. 

The subplot was probably also needed because the conflict between Lear and his two daughters had become static, a standoff. He is living in the wilds, and they had taken possession of everything he formerly owned. He refuses to go back to them under their terms, and they couldn't care less. Shakespeare may not have known where to go with that plot. Lear rages against his daughters, but he is helpless. His extreme rage is intended to explain why he doesn't just go back and live with them without his hundred knights. They would be glad to provide first-class shelter, since it must be an embarrassment to have their father wandering around in a condition worse than a beggar's. The introduction of the subplot renews dramatic interest, along with suggesting the universality of the theme.

The sub-plot mirrors the main plot.  Lear, excepting Cordelia, has unfaithful daughters; Gloucester, excepting Edgar, has an unfaithful son. Both plots describe the machinations the unfaithful children of the king and duke, Goneril and Regan disposessing Lear and plotting against each other, Edumund supplanting and smearing Edgar to inherit fully his father Gloucester's estate. As the two brothers, Edgar and Edmund are at odds, so are the two sisters, Goneril and Regan after Edmund starts a love triangle and causes them to do each other in over their jealousy for him.  An interesting parallel occurs after Lear is outcast and becomes more deranged. He, along with his faithful servant Kent, meet Edgar, who is playing the madman as a disguise.  Here we have two characters, both disposessed of what was rightfully theirs; Lear by his unfaithful daughters and Edgar by his brother. There's an interesting contrast in the actions of the siblings -- The evil sisters do themselves in; in the case of the Gloucester brothers, one kills the other, Edgar, the "good" son surviving.  Unfortunately, the "good" daughter, Cordelia, innocent to all the evil caused by her sisters, is killed as well. Edmund, therefore, wipes out Lear's progeny.

What is the role of the subplot in King Lear?

Our eNotes site on Lear provides an excellent and direct answer to your question. Referring to a literary critic, Ian House, it begins "House emphasizes the dynamic relation between the main plot and the subplot in King Lear, proposing that the differences as well as the similarities between them unsettle and illuminate our understanding of the principal story. As the critic explains, the double plot universalizes the action by shifting emphasis away from individual characters and situations; the effect is more like that of a prism than a mirror, multiplying images rather than giving back a single one. Further, House analyzes the notorious implausibility of dramatic events in Lear, arguing that the absurdity is purposeful and heightened by the changes in the humorous tone of the subplot "from farce to melodrama, from domestic tragedy to surrealism." In the course of discussing these issues, the critic provides extended evaluations of Gloucester, Edmund, and, especially, Edgar.]  click on the url below for the full discussion.

Traditionally, critics saw the events of the subplot to be repetitious of the main plot. The subplot deals with the tragedies of Gloucester while the main plot deals with Lear's problems. More recently, critics have been looking at the differences between the two plots. Most modern critics see the subplot as serving to intensify the main themes of the play, using it to further demonstrate such themes as the ingratitude of children and the idea of individual identity.

What does the above mean? Not all the critics agree as to the purpose of the subplot. I would suggest looking at the differences and similiarities of the two plots and drawing your own conclusions. Look at the central themes of the play and see how the events of each plot demonstrate those themes. When you get a question like this, where you don't have a clear answer, always cite evidence from the literature to back up your belief, and most teachers will accept your answer. I hope this helps in answering your questions.

How does the subplot support the main plot in King Lear?

Shakespeare's intent is to show how one generation gradually supplants the generation that bore and nurtured it. The subplot deals with meles, whereas the main plot deals with females. This suggests a universality to the theme. Keats speaks of "hungry generations" treading down the older generations in his "Ode to a Nightingale." This is really what King Lear is about. The characters and their stories are only rather extreme examples of a general rule. 

Furthermore, the subplot is really necessary because the main plot has more or less stalled. Nothing is happening. King Lear disowns both his daughters and goes out into the open country to live like a vagabond. His daughters make no effort to persuade him to return to the shelter they are willing to provide if he accedes to their terms. There is a standoff in the conflict between Lear and his daughters. This is dramatically dangerous. It risks losing audience involvement. This is where the subplot involving Gloucester and his two sons is needed. Edmund wants to take over everything his father owns, including his title. He writes his own sentiments in a letter but attributes them to his naive brother Edgar.

'This policy and reverence of age makes the world bitter to the
best of our times; keeps our fortunes from us till our oldness
cannot relish them. I begin to find an idle and fond bondage in the
oppression of aged tyranny; who sways, not as it hath power, but as it is suffered. Come to me, that of this I may speak more. If our
father would sleep till I waked him, you should half his revenue for
ever, and live the beloved of your brother.             I.2

Edmund's treatment of his father parallels what Goneril and Regan have done to Lear. Soon Gloucester will find himself out in the cold, just like Lear. There will be a very poignant meeting between these two old men in Act 4, Scene 6. It is both sad and funny. Life is a tragedy. Life is a comedy. It always has been. Like a lot of old men, Lear and Gloucester discuss the human condition from the point of view of old age.

Only the man who attains old age acquires a complete and consistent mental picture of life; for he views it in its entirety and its natural course, yet in particular he sees it not merely from the point of entry, as do others, but also from that of departure. In this way, he fully perceives especially its utter vanity, whereas others are still always involved in the erroneous idea that everything may come right in the end.
                                                                  Schopenhauer

Lear and Gloucester have lost everything. They have been pushed aside. Nobody cares about them. So they are both philosophical about life. They do not talk about their offspring, because their children are only examples of a general rule. They are more interested in the general rule itself. They talk about humanity as a whole and about the meaninglessness of life. This is where Shakespeare states his thesis, which is the guiding light of the whole play.