Dolphus Raymond explains to Scout and Dill that he acts drunk because it gives people a "reason" for his choosing to marry a black woman and to live with the black people. The rest of the white society still doesn't accept that he has made this choice, but they feel better being able to assume he lives this way because he is drunk and doesn't know any better.
In Bob Ewell's mind, he can save face if his daughter was raped by Tom Robinson. It is, obviously, a convenient cover for the fact that he (Bob Ewell) was the one who beat Mayella Ewell. It is also, to Bob, a better reason for the interactions between Mayella and Tom--Bob Ewell would rather that people think his daughter was raped than that she fell in love with/had a crush on (and kissed) a black man.