Why does Atticus sit outside the Maycomb jailhouse in chapter 15 of To Kill a Mockingbird?

The scene in which Atticus waits outside the jailhouse is an important window into the racist undertones of Southern culture in the 1930s. Furthermore, the scene is a potentially confusing one, especially the first time a reader encounters it. Upon subsequent readings, however, it's clear that Atticus sits outside the jailhouse in order to prevent a mob from murdering Tom Robinson. 

Harper Lee never comes out and says that the strange men are a lynch...

The scene in which Atticus waits outside the jailhouse is an important window into the racist undertones of Southern culture in the 1930s. Furthermore, the scene is a potentially confusing one, especially the first time a reader encounters it. Upon subsequent readings, however, it's clear that Atticus sits outside the jailhouse in order to prevent a mob from murdering Tom Robinson. 

Harper Lee never comes out and says that the strange men are a lynch mob, but the gradual tension of the scene reveals the deadly motive behind their visit. It's apparent that the men are drunk, and that they mean to harm someone. The brilliance of this scene is in the fact that Lee only reveals the mob's murderous intentions afterwards, by making it plain that they aimed to harm Atticus for getting in their way, and by referring to them as "a gang of wild animals" (160). As such, it becomes clear that, though Atticus was perfectly calm when confronting the men at the jailhouse, he was actually being very brave, as his presence (along with the presence of Mr. Underwood and his shotgun) was the only thing standing between Tom Robinson and a grisly end.