I don't have the book so I can't give a line of evidence but Atticus was all about teaching Jem and Scout those little life lessons without them even noticing so this instance was one of those little lessons he gives them. Hope this helps :)
Sacrifice... Not all heroes are successful for a while...
Explanation:
If teens aren't reading enough. Then their brains start to sort of lose smarts. So the more you read you learn information and it builds up personal experience. This answer make not make much sense but I tried.
