1. It forces you to write. There is a set word target so you can't just sit around with a blank page in front of you - there are only 30 days and nearly 2000 words a day are required.
2. It doesn't need to go anywhere. Whatever you write during Nanowrimo can be saved to your hard drive, it doesn't have to be a masterpiece - it can just be an extreme exercise class for your imagination.
3. Lots of writers support each other and remain in touch throughout Nanowrimo so it makes writing far less solitary. I remember spending early mornings sat in Starbucks with Lorrie Porter - she did less writing and more talking but still churned out an impressive word count and got exactly where she wanted to be.
4. It doesn't matter if you fail. There are no prizes (apart from a web banner) for completion so even if you could managed 10,000 words its still an accomplishment to be proud of.
5. It's like a little writer's holiday. You focus on something new and let everything else go on the backburner for a while. It's therapy.
I'm sure people can think of dozens more reasons why Nanowrimo is an excellent concept. I'd definitely recommend it to anyone who wants to try writing a book but knows that they'll probably type the first chapter and then give up (Samantha Williams).