The trial of Tom Robinson affected almost everyone in the town by requiring them to look at issues of race more critically.
When Atticus Finch decided to defend Tom Robinson, it turned the town of Maycomb upset down. Citizens were used to looking at things a different way. Whites were superior to blacks, and you never took the word of a black man over a white woman. Atticus changed all of that. He may not have ended racism forever in one full sweep, but he did get people to think about things.
We see the change through the eyes of young Scout. She does not understand why children and adults alike are insulting her and her father.
"Do all lawyers defend n-Negroes, Atticus?"
"Of course they do, Scout."
"Then why did Cecil say you defended niggers? He made it sound like you were runnin' a still."
Atticus sighed. "I'm simply defending a Negro- his name's Tom Robinson…. (ch 9)
The problem is not that Atticus is Tom’s defense attorney. Of course white lawyers have to defend black men in court. The difference is that a black man is accused by a white woman, and Atticus will try to prove him innocent. The fact that Atticus will try is too much for people.
Atticus explains that he knows taking the case is going to make waves.
"Before I'm through, I intend to jar the jury a bit- I think we'll have a reasonable chance on appeal, though. I really can't tell at this stage, Jack. You know, I'd hoped to get through life without a case of this kind …” (ch 9)
It is not just the jury that Atticus is jarring. He is jarring the whole town. Blacks and whites alike will have to think about things more carefully, and the ugliness of racism, something no one talks about, will be out in the open.