Jaggers remains in his profession because he makes a lot of money at it. He has a special talent for defending criminals, and he is famous for his ability to get guilty persons acquitted. Jaggers, like many another criminal attorney, does not seem to care very much whether his client is guilty or innocent. He enjoys the challenge of getting a defendant acquitted even if there is strong evidence against him. He does not have to like his clients, any more than a surgeon has to like his patients. In fact, Jaggers obviously dislikes many of the people he has to deal with, but he will usually defend them if they can afford to pay his fee and if he thinks there is at least a fifty-fifty chance of getting them off.
Pip first sees Jaggers in action in Chapter 20, where the attorney is besieged by clients and would-be clients who obviously look upon him as their potential savior. Jaggers shows his character and his motives in his first words:
"Now, I have nothing to say to you," said Mr. Jaggers, throwing his finger at them. "I want to know no more than I know. As to the result, it's a toss-up. I told you from the first it was a toss-up. Have you paid Wemmick?"
Jaggers is the most interesting and amusing character in the novel. He is an obvious caricature of criminal attorneys in general. If a client is actually guilty, he doesn't want to know about it, because he is technically a representative of the court and has to divulge anything he knows--but cannot divulge anything he does not know. If somebody is bringinig in a false witness to substantiate an alibi, that is perfectly all right with Jaggers--as long as he doesn't know about it. His last words to the two men he is addressing in the above quote show where his interest lies. He asks, "Have you paid Wemmick?" It would be a rare event if Jaggers ever took a case pro bono.
It is because Jaggers is a man who is able to keep his knowledge compartmentalized that he can represent Pip for years as his guardian without ever hinting that Pip's benefactor is Abel Magwitch in Australia. And when Magwitch returns illegally to England after having been "transported," Jaggers pretends to believe that the ex-convict is in Australia although he knows perfectly well that he is back in England at the risk of his life. Jaggers can also allow Pip to continue to believe that it is Miss Havisham who is his benefactor right up until the point where Magwitch reveals himself to Pip in Chapter 36.
Jaggers has a jaundiced attitude towards humanity. He has seen so much of human wickedness that he is actually a connoisseur of the worst specimens of mankind. He takes an immediately liking to Bentley Drummle when Pip brings him to dinner, because he sees the as yet not fully developed evil in this boorish young gentleman.
Jaggers is a pivotal character in the novel because he represents both Magwitch and Miss Havisham. It turns out that he was also responsible for placing Estella in Miss Havisham's care. And, of course, he is acting as Pip's guardian through much of the novel. Jaggers knows more than any other character in the book, but he is able to keep his knowledge in separate compartments because of his natural genius and his experience with the realities of life and the realities of the judicial system.
He probably needs money.