Abel Magwitch is important in the book Great Expectations because he, probably more than any other character, helps elevate Pip to a higher social status than Pip likely would have been able to reach on his own.
Early in the book, Pip shows Magwitch great kindness by bringing him food. Magwitch is a dirty slob of a man. He's rough around the edges, and a hardened criminal of sorts. The fact that Pip shows him kindness has a deep impact on his attitude toward Pip. Magwitch makes the decision to devote himself to making a fortune and using it to raise Pip to a higher social standing.
Magwitch teaches himself to read and write in order to help himself make more money to turn over to Pip. He becomes Pip's secret benefactor. He funds Pip's education in London. He funds Pip's lifestyle. He hires the lawyer Jaggers to help supervise Pip's transformation process. When it comes down to it, Pip doesn't become a gentleman without Abel Magwitch.