Boo Radley serves an important purpose in this novel. Throughout the book, the kids watch the prejudice and judgment of the adults. What they fail to realize is that they have done the same thing with Boo Radley. They have believed rumors about him. They have done things that could possibly hurt him. All Boo did in return was show them love, kindness, friendship, and saving grace. Sure, Boo was very different from the kids. For some reason, he really doesn't like to be around people, but he was as much of a friend as he could be to them.
The black community had never done anything wrong to the white community, yet the white community judged them, particularly Tom, to an extreme.
Without Boo, Atticus' ongoing message to his children to respect all people for who they are would not have worked or been something that his kids could relate to.