Scout and Jem (and Dill) sneak out of the house to find what Atticus was up to because they never see him take the car out, especially at night and with a light.
They find Atticus and some men have driven up to talk with him under Tom Robinson's cell at the jail. Because the kids go up to see Atticus, the purpose the men have in being there is unable to transpire. They wanted to mess with Tom at the very least. Had the kids not shown up who knows what they would have done. These grown men didn't want to fight Atticus in front of children because it is inappropriate to do so. They had at least that many manners. Scout began talking to Mr. Walter Cunningham about his son and essentially befriended Mr Cunningham. Had she not done this, the men would have done something terrible to Atticus and/or Tom. Scout's friendship made the entire situation go away.