What is the dramatic significance of the subplot in King Lear?
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.