Yup, exactly what arrowchild said! It's pitting yourself against yourself - it's the challenge of putting aside an hour or so a day for 30 days to write a little over 1000 words each day. It's especially great for people who need motivation: the people who say "well, I'd love to get this idea out but x,y,z" because it gives you a goal and motivation. I did it last year when I was out of work and it was a great challenge and escape. This year's been fun because I am working now so I have to plan better.
But I also find planning/scheduling fun so I'm just a bit weird there