I ask myself this question often, especially at this point of every semester. At least I've completed my take-home final and am closing in on completing the research paper. This class wasn't even as writing-heavy as some of the others.
But still, I ask myself the question. Yes, there is the keeping-myself-busy explanation. But there are many ways I could do that. I guess it's because I want to keep learning, and I learn best when I write things down. In the end, it's as simple as that.