Being a workaholic, finishing a chapter means I get to put down the pen and go do something just for fun - like watch a movie, or read a book, or play videogames for a while. I have to stop myself from immediately starting something new - because it's highly likely that I will, and forget about taking breaks and being nice to myself altogether.
Also, taking a break means I clear out my brain a bit, which means that when I come back and pick the pen back up, I'm feeling refreshed and more creative. Getting a bit of distance also means I can look over what I've drawn and see if anything needs to be changed.