why did the author put Mrs. Dubose scene in the book?

Atticus sees what most people do not in Mrs. Dubose. Where people see a morphine addicted nasty old lady, Atticus sees a lady who had a difficult life trying to die with a shred of dignity. One of my favourite lines from Atticus, and there are many, is when he tries to explain this to his kids,

"I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. Mrs Dubose won, all ninety-eight pounds of her. According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew."

Mrs. Dubose was a lonely old lady who numbed the pain of many years the only way she could. In the end she battled her addiction and found room in her heart to let two children, Scout and Jem, into her life.