Good question. Eyes and all variations on this—physical sight, literal sight, vision, etc.—symbolize clear understanding. In other words, physical vision stands in for spiritual and emotional vision. Lear's great failing early on in the play is that he cannot "see" that Cordelia's love is the truest of all his daughters. The evil characters permanently blind good men, while good characters like Kent briefly take on disguises in order to do good works. But in general, eyes are true vision, and so intensely important and intensely vulnerable.