We've read pretty much everything that Beckett has ever written, and his characters' different physical handicaps still have us pretty confused. Consider what we have in Endgame. Hamm is blind and in a wheelchair. He gets headaches. Nagg and Nell have no legs, and they are unable to see each other. Nagg is hard of hearing. Nell is unable to cry. Clov has stiff legs and is unable to sit down. Why is everyone so physically impaired?
There are plenty of explanations, but here's just one. When Clov says that there is no more nature, Hamm says, "But we breathe, we change! We lose our hair, our teeth! Our bloom! Our ideals" (1.107). In the play, growth seems to be measured in terms of decay. Having a body seems to be a very undignified thing, and it brings all sorts of disgrace upon us.
Now, consider the fact that, before James Joyce's Ulysses, the body was something that wasn't exactly celebrated in English 19th century literature. A writer like Jane Austen could appreciate beauty and good dress, but no one ever burps or passes gas or pees in one of her books. James Joyce violently broke this tradition in the English novel by introducing his hero, Leopold Bloom, as someone who poops and masturbates. The body is no longer something to be hidden: it's something to be celebrated.
Now, Beckett, who was friendly with Joyce, takes this innovation and points out that it's not exactly a celebration. Our bodies fall apart. We get old, our skin wrinkles, and our hair falls out. In our minds, we imagine ourselves as being extremely dignified, but the body is something that can constantly undermine this feeling of dignity. By exaggerating this so strongly, Beckett draws special attention to the broken bodies of his characters. It is yet another aspect of their misfortune, something that they have to struggle against.