Scout's insufficient understanding of the tremendous tension of the situation at the jail points to her familial intuition that propels her toward intervening for her father. Thus, in Chapter 15 Harper Lee underscores the importance of love and family as well as common sense.
Harper Lee, in this chapter, uses a limited narrative perspective to great effect. Readers are aware of the mob's intentions (to lynch Tom Robinson) but Scout is not.
The discrepancy between these perspectives creates tension. The reader waits for (and anticipates) Scout's realization of the nature of the situation she has stepped into. It doesn't hit her until the next day at breakfast, but the scene has power and tension nonetheless.
This chapter is a great one because it demonstrates how the child can shame the man. As the saying goes, "out of the mouths of babes." This means that children say the darndest things, but often they hit it right on. Scout's naive attempt at helpfulness disarmed the men and made them realize what they were doing.
It should be obvious to most readers (adult, at least) that the groups of men arriving at the jail by automobile are not there for chit-chat with Atticus. When the men brag about drawing Sheriff Tate away from the jail on "a snipe hunt," it is clear that their intention is to take Tom from his cell. They are just seconds away from doing just that when Scout makes her appearance. She is clueless as to their true motives, believing that Atticus was "expecting them." Only when she sees them up close does she realize that most of them are strangers. She doesn't understand why Jem seems so serious, refusing Atticus's demand that they leave. It was not until the next morning that "the full meaning of the night's events hit me and I began crying." Scout's narration comes from the eyes of a child and not from the future adult perspective which is found often in the novel.