Charlie enters the book in the second-to-last chapter and plays a small but pivotal role. Aside from us readers, Charlie is the only person to hear Jacob's whole life story. We are the chosen ones, Charlie.
Unlike anyone else in the modern section of the story (other than perhaps Rosemary), Charlie understands that Jacob's history is valuable. As Charlie tells Jacob, "Mr. Jankowski, […] it would be an honor and a privilege if you would join me for a drink in my trailer after the show. You're a living piece of history […]" (24.44). Charlie proves to be an awesome listener, which encourages Jacob to reveal long-hidden secrets.
Charlie also proves he's a quick thinker who doesn't mind lying to the cops. When a policeman shows up looking for Jacob, Charlie covers for him and says Jacob is his father. Jacob is hoping and praying that Charlie will let him stay with the circus, and it seems like Charlie can read his mind. Against his better judgment, perhaps, Charlie invites Jacob to stay. By doing so, he seems to give Jacob a new lease on life and help him feel like he's finally found a place where he belongs again. Jacob has barely met Charlie, but it seems like the two are closer than Jacob is with his own son. Charlie might even represent all that Jacob used to be. Pretty cool way to end a story, we think.