I might not have written any novels if it weren't for National Novel Writing Month. Back in 2002, someone in my writing group said, "There are these people trying to write 50,000 words in November. We should do it." It sounded impossible. It sounded crazy. Eight Novembers and eight novels later, I'm here to say that writing a first draft in a month is possible, only partly crazy, and a wonderful and valuable experience.
In the cult of NaNoWriMo, all that matters is the word count and the deadline. To win, you have to write 50,000 words during the 30 days of November. You don't have to write anything good. You don't have to show it to anyone else. You don't get a prize, except for a downloadable certificate, bragging rights, a feeling of accomplishment, and oh yeah, 50,000 words you wouldn't have written otherwise.
The purpose of the exercise is to force yourself to write -- lots and quickly. 50k in 30 days is an arbitrary goal, but thanks to the peer pressure of over 150,000 other participants worldwide, it's not hard to convince yourself that it really matters. If you can buy in completely and break free from the inner editor that censors your words, you're in the perfect mental space to create a first draft. Writing at speed and thinking less leads to all sorts of brilliant scenes and creative twists that you never could have come up with through careful planning. It also leads to a lot of garbage, but it's simple enough to take that out later.
All first drafts are full of garbage. The plots are confusing, the character motivations are weak, and the pacing is way off. These problems are as likely if you take years to write a first draft or only a month. I've tried both ways, and I prefer to get it over with as quickly as possible.
Once you have a story down beginning to end, flawed as it may be, you have something whole to work with. I don't like the idea of revising a novel that's only partially written, and I know you can't edit one that doesn't exist yet. That's why I highly recommend NaNoWriMo for anyone who wants to write a novel but can't get started. Commit to the ridiculous premise, and before you know it, you'll have not only started, but you'll be done. And I guarantee you'll be amazed.