There are three reasons.
1) If you don't do it now, you probably never will. Here's the truth: 99% of us, if left to our own devices, would never make the time to write a novel. The structure of NaNoWriMo forces you to put away all those worries and START. Once you have the first five chapters under your belt, the rest will come easily. Or painfully. But it will come. And you'll have friends to help you see it through to meet your word-count goal.
2) Aiming low is the best way to succeed. Once you start evaluating your story in terms of quantity instead of quality, you take that pressure off yourself. And you'll start surprising yourself with a great bit of dialogue here and an genius plot twist there. Characters will start doing things you never expected, taking the story places you'd never imagined. There will be horrible writing, yes. But along with the bad writing, there will be beauty. A lot of it.
3) Making art for fun does wonderful things to you. It makes you laugh. It makes you cry. It makes you want to take naps and go places wearing funny pants. Doing something just for the heck of it is wonderful medicine.
