What could the background description about the Ewell family have to do with this trial?

People already knew what the Ewell family was like. The whole town knows Bob Ewell is a drunk and likely abuses his daughter in the worst ways. Ordinarily this background knowledge might work against Ewell in a trial. The accused, however, is a black man and the white jury is unwilling to take the side of a black man even though the accuser is Bob Ewell.