A funny thing happened to me on the third day of November: I realized I was writing the wrong book. My heart wasn't in it. So four days late, I began writing something new. I'm still behind on my word count due to this switch, but now I'm writing something I know I can finish. Why does that matter? I'll tell you why.

Finishing is the most important part of writing a novel. Period. Sure, there are many important pieces to a novel. But if you never reach the end, none of those other things will ever matter. I first participated in NaNoWriMo back in 2004. I'd toyed around with writing a mystery novel before, but it was NaNoWriMo that made me type "the end" for the first time.
If you live a normal busy life, there's never a good time to write a novel. In November, you have the collective energy of thousands of other people around the world doing the same thing. If you want to write a novel, now's the time to try it. If you signed up as an official participant, you even receive helpful pep talks.
It's good to learn the lesson "Don't get it right, just get it written." When I first started writing, I waited for the right words to come to me. That's a surefire way to finish one book every twenty years. It's a hell of a lot easier to edit a bad scene into a good one if you've already written down a cringe-worthy rough draft.
It's also good to learn how to write on a deadline. If you want to write professionally, you have to learn to do this. It's hard to give yourself a deadline, so NaNoWriMo gives you one.
You never know what might happen with this novel after November. The first novel I completed for NaNoWriMo back in 2004 was the one that went on to win writing competitions. If I hadn't discovered NaNoWriMo, I don't know that I ever would have finished a novel. Maybe you're the rare person who can motivate yourself, but for the rest of us NaNoWriMo is a great tool.