Magwitch is Pip's benefactor.  He is also referred to as Provis near the end for security reasons.  After he finds out this information, he also finds out that Estella is his daughter (Molly is the mother).  Pip realizes what a good man Magwitch truly is.  He sacrificed all of his earnings to give them to a little boy who helped him in a time of need.  Pip even went out of his way to get a file for him to escape the leg iron.  When Pip sees the sacrifices Magwitch made for him, he realizes that he was never going to be a gentleman worthy of Estella.  He has nothing but a blacksmith's background and a convict's earnings.  So at this point he knows that he will "get the girl."  He also realizes at this point that all he did to hurt Joe was for nothing.  He deserted the one man who loved him and wanted only the best for him.