What is the importance of Boo Radley in the story "To Kill A Mockingbird" by Harper Lee?

Boo Radley is a subtle, yet very important character to the structure of the story of "To Kill A Mockingbird". We can learn a lot about him by regarding him from the viewpoint of several other characters in the book.
His own family, the Radley family, are ashamed of him and disgraced by him. When Boo was involved in some 'antics' as a fifteen year old, Mr. Radley refused to send him to the state industrial school even though it wasn't considered a disgrace and was certainly better than prison. Mr. Radley instead kept him at home and didn't release him from his improvised prison. "....…