I think he owes someone something. Atticus loves Scout and Jem so why would he put them through it if he didn’t have to? He wouldn’t put them in danger so why take the case, other than the fact that if he didn’t no one would. Maybe he is trying to teach Scout and Jem that just because your white doesn’t mean your right. or maybe he is hiding something and has to be Tom’s lawyer because he owes him. It would probably be worth it because some people might see whats going on and actually want to change the way they look at the world, including the children.