Gloucester's loss of vision gives him clarity...

King Lear, IV.I (19-24)

Old Man: You cannot see your way.

Gloucester: I have no way, and therefore want no eyes;
I stumbled when I saw. Full oft ‘tis seen
Our means secure us, and our mere defects
Prove our commodities. Oh dear son Edgar,
The food of thy abused father’s wrath,
Might I but live to see in my touch
I’d say I had eyes again.

When Gloucester still has his eyes, he is blind to his two son’s true intentions. Gloucester is ignorant and foolishly falls for Edmund’s jealous scheme and casts away his true and loving son and heir, Edgar. This is why he says, “I stumbled when I saw”. Gloucester is crazy to assume such rash and unrealistic thoughts about his loving and loyal son, Edgar. This parallels the relationship between Cordelia and Lear; both fathers are blind to which of their children really love them until it is too late. After Gloucester has lost everything, including his eyes, he realizes his horrible mistake at falling for Edmund’s trick and rejecting Edgar. Ironically, it is Gloucester’s loss of vision that allows him to finally see. The Old Man he is talking to in this passage is actually Edgar in disguise so at least before the play is over he is able to tell Edgar how sorry he is for mistake and to tell him of his love for him as a father.