There are some obvious parallels between the main plot of the play featuring Lear and the subplot featuring the Duke of Gloucester. Both begin in a high station, but fall into ignominy and despair, brought down in the first instance by their own children. Lear is cast out by his wicked elder daughters, in whom he trusted; Gloucester is tricked by his villainous illegitimate son Edmund. Both Lear and Gloucester are deceived about the children who truly care for them - Cordelia in Lear's case and Edgar in Gloucester's - and in fact go so far as to banish them. Both pay dearly for such actions, although they are also reconciled with their loyal children before death.
As can be seen, then, the plots involving the two men are in essence, the same. To begin with, they both think they can command and are confident of their high position, but throughout the course of the play they are humbled; indeed, all their assurance is stripped away. In fact they have to endure horrific ordeals before they can, in the end, regain some measure of dignity and balance. Perhaps what is most important is that they learn some extremely harsh moral lessons, although the suffering that is inflicted upon them generally appears rather too extreme. Lear loses his wits; Gloucester loses his eyes.
Of course, the blinding of Gloucester is an important parallel to the metaphorical blindness of Lear. At the beginning of the play Lear's metaphorical blindness is considerable. Not only does he fail to recognize the goodness of Cordelia, the very act of dividing up his kingdom between his daughters simply on the public declaration of their love for him, is foolish in the extreme. Genuine love cannot be quantified in this way. Gloucester, too, is metaphorically blind to begin with, as he so easily falls victim to the unscrupulous Edmund's schemes. Only after his literal blinding does he gain wisdom: 'I stumbled when I saw', he remarks (IV.i.19)
Although the story of Lear does remain the main focus of the play, the sub-plot involving Gloucester serves to heighten and further dramatize the play's main themes. In fact, it is given to Gloucester, rather than Lear, to make perhaps the single most pithy remark highlighting the sense of upheaval, despair and almost unremitting tragedy of the play:
Tis the times' plague
When madmen lead the blind. (IV.i.47)