As a past "winner" of NaNoWriMo, I can tell you what this did for me.
- It set a lofty yet achievable writing goal for me to reach for
- It forced me to write, even when I didn't feel motivated (or when my inner-muse decided to sleep in)
- It put me in a community of others who were pursuing the same goal. This provided both support and encouragement.
- It provided me an excuse to write.
- It helped me as a writer, turn off my critical editor and to write, by imposing a looming deadline. Often, when writing smaller blocks, I would word-smith it to death and never finish.
- It lightens up the work of writing by infusing it with an air of excitement and fun.
- It built up my confidence by me writing that many words in such a short amount of time.
I thought of a few more.
- Having a defined goal made it more difficult for me to quit, especially after I told my friends I was doing it.
- It made it easier to take time to write away from my family because they knew it was a real thing AND that it was time-limited.
- It made me write past writer's block and continue to do the hard work of writing in order to make the goal.
- It made me become disciplined with my time in order to have time to write.