Who is The Old Blind Piano Player from Cities of the Plain and what is their importance?

John Grady meets the old man after visiting Magdalena at the White Lake. Grady later asks the old man to be Magdalena's Godfather. The blind man declines becoming Magdalena's godfather and tells a story to illustrate the reason for his refusal. The blind man also talks about blind destiny and how strange it is to call destiny blind. "...what sort of destiny is that?"