Essentially, Pip's illusions about class are given a good crack here, because he learns his benefactor was not the elegant, snobby Miss Havisham, but Magwitch the convict.
One of the convicts says he's been delivering money to a fellow named Pip and that Magwitch is giving him this money as payment for "service," that is, the child Pip's helping Magwitch by getting him food and a file at the beginning of the novel.
This revelation relates to the novel's themes about class and character. For a long while now, ever since he set his heart on winning Estella's affections, Pip has been a snob, shunning his low-born loved ones and spending more money than he has to appear a gentleman. He's assumed it is Miss Havisham who's been giving him money, but instead his benefactor is a criminal. Yet the low-born criminal seems to have more compassion and generosity than the "well-bred" old woman.
This is an important part of Pip's development, as he must cast off his snobbery before he can truly grow and mature as a person.