When Jem and Scout get attacked by Mr. Ewell, it isn't their father or Mr. Tate that saves them. But it's Boo Radley, the person they were afraid of, interested by, and now saved by. As a reader you don't expect to see a theme based around a character you don't see until the end of the story. But because the book was written this way it makes the biggest impression, it shows that people don't need the attention or even to leave a house to be a good person. All it takes is the willingness to help someone in need.