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.