Snagsby assigns the freelance legal copying work Nemo was doing before he died. He's a small, generous, compassionate, and totally confused guy who just wants to be left alone.
If Jarndyce is the novel's uber-giver, Skimpole the uber-taker, and Mrs. Jellyby the uber-head-in-the-clouds-idealist, Snagsby might actually represent the novel's ideal of generosity for a person of his relatively modest means.
Snagsby has two things going for him: he is somewhat dim and he's very good-hearted. These qualities help prevent him from falling into the evil web of Tulkinghorn (mostly because he doesn't really get what Tulkinghorn is all about), and they help him react appropriately to the various unfortunates he meets in his life. At home, half his time is spent taking care of his servant Guster, who has constant grand-mal epileptic seizures and probably would have been out of a job in many other houses. When he's out and about he meets, is nice to, and gives money to Jo, who calls him "Sangsby" and talks about him in almost the same worshipful way he talked about Nemo.
Snagsby doesn't wear his generosity on his sleeve and doesn't try to capitalize on it for his reputation like Mr. Chadband does. Nor is what he does useless, like Mrs. Jellyby. He doesn't manage to save Jo, but it doesn't seem like anyone could, and at least he makes the boy's life a little more tolerable. As for Guster, where else could someone like her find a home, especially in those days?