Why does Dickens make Magwitch the benefactor?

I wouldn't presume to guess Dicken's intentions, but Magwitch's identity as the benefactor is the reason Pip is able to reevaluate his position, the way he attained it, and the place he came from. Pip's feelings of entitlement, and his belief that his benefactor (benefactress) has purposely set him up for a specific reason (Estella) set him on the course he desires at the expense of everything and everyone he's known. Finding out that his benefactor was Magwitch changes everything, and spending time with Magwitch opens his eyes.