What makes Mr. Jaggers a pivotal character?

Mr. Jaggers is one of the most memorable characters in Great Expectations. He is the quintessential lawyer. He will represent anybody as long as he gets paid. In criminal cases he doesn't care whether the client is guilty or innocent. He only cares about getting his fee and being able to win the case. If he considers a case unwinnable, he won't take it. He is especially characteristic of the class of lawyers and especially funny in not wanting to know anything he shouldn't know. It takes a lawyer like Jaggers to represent a transported convict like Abel Magwitch. Jaggers assumes that Magwitch intends to return to England, which is a hanging offense; but as long as Jaggers hasn't been "informed" of that intention he is able to remain professionally ignorant. When Magwitch actually does return, Jaggers doesn't want to hear about it or know about it. As far as he is concerned, his client is still living in far-off New South Wales.
Jaggers is described as a very big and imposing-looking man. Pip first encounters him at Miss Havisham's house and describes him as follows:
He was a burly man of an exceedingly dark complexion, with an exceedingly large head and a corresponding large hand. He took my chin in his large hand and turned up my face to have a look at me by the light of the candle. He was prematurely bald on the top of his head, and had bushy black eyebrows that wouldn't lie down, but stood up bristling. His eyes were set very deep in his head, and were disagreeably sharp and suspicious. He had a large watchchain, and strong black dots where his beard and whiskers would have been if he had let them. He was nothing to me, and I could have had no foresight then, that he ever would be anything to me, but it happened that I had this opportunity of observing him well.
Dickens makes Jaggers physically imposing to fit his domineering manner and his amazing ability to hold all sorts of information in separate compartments inside his great head. He is a very important character in the novel because he is the sole connecting link between so many of the important characters. Only Jaggers could be a link between such people as Miss Havisham and Abel Magwitch. She knows nothing about Magwitch and he knows nothing about her. Yet she would never have gotten Estella as an adopted child if Jaggers had not been representing both her and Magwitch. In that case, Pip would never have met Estella and fallen in love with her. His whole life would have been different. He might not have been so strongly motivated to become a gentleman. Estella, too, would have had a very different life and a very different character.