It's late, it's been a busy day, so why bother to spend 10 or 15 minutes at night jotting things down?
For me there are a few reasons, but the main one is that it's cathartic. I can unwind a little, let go of things, and sometimes think more clearly. When I was doing more authoring, a little writing after work helped to cement some ideas in my head, and many times gave me something to ponder or work on overnight or throughout the next day.
Taking a few minutes to write usually just makes me feel better, and better understand what's happened in my day.