What does the letter Pip receives in the middle of Chapter 52 tell Pip to do? Why does he do it?

Wemmick sends Pip a note indicating that now may be a good time to escape with Magwitch and get him out of the country. Herbert and Pip plan to take the boat out with Magwitch in a few days, take him down the Thames until they run into a steamer headed for a foreign port. In the meantime, Pip gets another letter, this one by an anonymous author, telling him to come down to the limekiln in the marshes that night. Once again, Pip goes to his hometown and walks out to the marshes. Joe truly was his benefactor and never asked for anything in return (unlike Pumblechook). With his reflections on Joe and on Magwitch, Pip is now examining the great relationships in his life as opposed to his great expectations.