Mr. Dolphus Raymond knows that in Maycomb there is a certain culture with its customs and rules. By pretending to be a drunkard, he provides the white people an explanation for what they consider his scandalous behavior of cohabiting with a black woman and having biracial children. By his being nothing more than a drunkard, the townspeople can dismiss him into a lower category of their society than the one he is really from, and not be so upset by his "out of line" behavior. This need to categorize people will again be roused by Tom Robinson's stepping "out of line" and becoming involved in the life of a white woman.
When Tom Robinson is brought to trial and found guilty, it is not so much that the jury has believed the testimony of the reprehensible Bob Ewell as it is that Tom has stepped "out of line" and outside his social status in Maycomb by talking with a white woman and, especially, by feeling "sorry" for her. As Mr. Underwood remarks later in his editorial on Tom's death, Tom ran because he knew he was condemned, not by any of his actions, but by the social measurements and boundaries of the "secret court of men's hearts."