In Ch. 15 of To Kill a Mockingbird, why does Atticus wait outside of the jailhouse?

Tom Robinson is moved to the jailhouse prior to the trial and Sheriff Heck Tate warns Atticus that there is a group of men who are unhappy about it, who have been drinking, and who might plan on making trouble. What he really means is that these men might well end up going to the jail and hurting Tom Robinson. At first, Atticus does not believe this is a possibility.

"Don't be foolish, Heck," Atticus...

Tom Robinson is moved to the jailhouse prior to the trial and Sheriff Heck Tate warns Atticus that there is a group of men who are unhappy about it, who have been drinking, and who might plan on making trouble. What he really means is that these men might well end up going to the jail and hurting Tom Robinson. At first, Atticus does not believe this is a possibility.

"Don't be foolish, Heck," Atticus said, "this is Maycomb."

But Tate argues that it is not the men of Maycomb, but a group from "Old Sarum," who are getting upset.

So, in order to be a visible show of protection for Tom Robinson, Atticus goes to the jail and sits outside. It turns out Sheriff Tate is correct and the mob ends up confronting Atticus. Fortunately, his kids and Dill show up and Scout diffuses the whole situation by striking up a conversation with Mr. Cunningham. After the mob leaves, without doing any harm, it's revealed that Mr. Underwood was watching them, with his shotgun at hand. This proves how real the threat was.