As was mentioned in the previous post, Atticus sits outside of the Maycomb jailhouse in order to make sure Tom Robinson is safe. At the beginning of Chapter 15, a group of concerned citizens visited Atticus at his home to warn him about the Old Sarum bunch. They believed that the Old Sarum bunch would attempt to lynch Tom Robinson before the trial. Later on that night, Atticus drives up to the Maycomb jailhouse and sits out front of Tom Robinson's cell and reads his newspaper. Jem, Scout, and Dill end up looking for Atticus and find him quietly reading outside of the jailhouse. Shortly after, the Old Sarum bunch arrives and confronts Atticus. Atticus displays courage by refusing to leave. Scout and the children end up walking out from their hiding place and Walter Cunningham, the leader of the mob, ends up telling his men to go home. Atticus' decision to sit out front of the Maycomb jailhouse saves Tom Robinson's life the night before the trial.
Atticus sits on a chair outside the jailhouse in order to make Tom Robinson feel more secure and safe. Atticus has an air of leisure...reading the paper, but obviously expecting visitors.
A group of men come to the jail and explain to Atticus that they have come for Tom. Unbeknownst to Atticus, Scout, Jem, and Dill are hiding in the woods watching the entire scene. Scout, scared for Atticus' safety, runs to his side. She realizes too late that she has come among men she does not know. However, when she recognizes Mr. Cunningham and speaks of the goodness in his boy, Walter, and how Atticus has helped the Cunninghams in the past, she unwittingly changes Mr. Cunningham's mind about the way things should go that night. He instructs the mob to leave.
So, to make a long story short, Atticus sits out in front of the jailhouse for two reasons: one, to comfort Tom Robinson in his hour of need, and two, because it is the right thing to do. It is not a black or white thing--it is the simply the right thing regardless of race, religion, gender, sexual orientation, blah, blah, blah. Tom Robinson is not just a black man. He is a human man who deserves a fair trial...not a midnight lynching.