Not good writing, mind you, just writing. NaNoWriMo is not about quality, it's solely about the word count. You get a certificate if you reach the target, and that's about it.
So why do it?
One of the great things about writing is that it's a process. You start with some raw material and you then hone and polish them into something worth reading. Everybody has to do some editing; you're one in a million (a billion?) if you produce something worth publishing on the first draft. But you need the raw material to start off with. NaNoWriMo could be the spur that gets you producing the words, and there's certainly lots of support from other NaNoWriMo writers, both on the forums and in local groups.
I tried it last year. To be honest, I thought it would be a bit of a doddle because I'm quite good at regularly producing a couple of thousand words per day. I thought it would spur me on to produce those couple of thousand every day, instead of perhaps 4 days out of 7. I loved the idea of having 50,000 words to play with at the end of the month - what a Christmas present to myself!
What happened was it became yet another thing to feel guilty about, another tool to beat myself up with. I found I was producing less and less. 'YOU MUST WRITE' pulsed in my head until it throbbed and the last thing I felt like doing was turning on my laptop. December came as a huge relief.
I'm now a bit wary of the whole shebang. It suits some personalities, but not others. I suspect that those people who love it are the sort who find it quite easy to churn out thousands of words anyway. They can revel in the luxury of a copious output and know they will be envied. Those of us who love the editing, rather than the first draft bit, just get depressed because NaNoWriMo isn't about our sort of skillset. Tortoise or hare, horses for courses - enjoy, or avoid, it's up to you.