What is Lee's purpose for introducing Mrs. Dubose?

Miss Dubose is a lesson in both courage and tolerance. She is an old lady who is slowly dying. She is determined to break her addiction to pain killers and die naturally. She wanted to die with dignity, for her that meant no pain killers. Much of her venom comes from her physical and emotional pain. Atticus knows this. Atticus sees a similar courage in the old lady as he has by taking the Tom Robinson case. Atticus sees this courage and attempts to teach tolerance and understanding in his kids using her as an example.