Most of my understanding comes from friends who do NaNo -- it's a challenge in discipline! And it's definitely not for everyone.
But sitting down and saying IM GONNA WRITE 5000 WORDS TODAY is a way to keep yourself from going back and editing and second-guessing and dithering, and just finish the dang thing. By the end you've a novel's length story!
It's sort of similar to Hourly Comics Day or 24-Hour-Comic Day. Trying to finish a 24 page comic in 24 hours isn't, like, a good standard to make a comic, but it's fun for some people to try it as a challenge. Stop spending ten billion hours on this page and just make it as quickly as you can. Stop doubting and second-guessing yourself. There's not time for that! Just do it!! You might be surprised at how much you can accomplish in that short time frame!
Pushing yourself constantly is bad, but pushing yourself briefly for a challenge is, for some people, rewarding because it shows you how much you're capable of. Even if you can't finish, pushing yourself up to your limits shows you where those limits are, and they might be further than you thought!
Of course the Actual Novel You Care About And Are Putting Your Best Into isn't gonna happen in a month, but once you've gotten in the habit of 2500 words a day from this challenge, you learn how much you really can do. You have the confidence and practice of someone who's finished a huge portion of a novel before.
For other writers, it's just stressful. It doesn't mesh well with how they work, or how they think, and it isn't rewarding, so they don't do NaNoWriMo. It's completely possible -- and it's not a bad thing! -- for a lot of people to click with something that just doesn't click with you.