The first question you ask is such a good question that it is almost impossible to answer. We might say, why does anyone do anything?
However, the second question cuts to the heart of the story, and that one we can and should answer. In addition to being a great (Southern) writer, O'Connor was two other things: she was a devout Catholic, and she was someone suffering from a terminal disease. Her lupus killed her before she reached 40, and her father had fallen sick with lupus when she was about 12 and died when she was 15. She suffered, and knew she suffered without real cause. Things just happened, and things just happen to people in her fiction, as in this story. At the same time, her faith leads her to see grace and meaning in things, and that's visible here. O'Connor was part of our degraded world just as Mr. Shiftlet was in the story, and both of them cry out for grace through their words.