Why do you think Lee chooses to use Scout to stand between the mob and Atticus in To Kill a Mockingbird?

In Chapter 15 of To Kill a Mockingbird, Scout and Jem find Atticus at the Maycomb County jail standing guard over Tom Robinson when an angry mob appears, probably with the intent of removing Tom from his cell and possibly harming or even lynching him. Scout's presence makes the mob disperse. She tells Mr. Cunningham, a member of the mob, to say hello to his son, Walter, who goes to school with her. She also reminds Mr. Cunningham that Walter came to dinner at her house. Mr. Cunningham responds to her,  “I’ll tell him you said hey, little lady” (page numbers vary by edition), and then he commands the mob to go home.

Lee chooses to place Scout between Atticus and the mob because Scout is a symbol of innocence. As a young child, she doesn't quite understand the racism and injustice that would cause a mob to try to lynch an African-American man who is waiting to stand trial. In front of her, Mr. Cunningham is reminded that he is a father, just like Atticus, and he is embarrassed by his evil intentions. In the face of innocence, the mob backs away and leaves the jail.