Partly there's a very good, supportive regional group here; more than that, though, I discovered the first time I tried it that the combination of freedom (no editting, no worrying about quality) and discipline (1667 words a day, and omg do Virgos like me use that word count app seeing how far we've gone after each sentence) really works for me. Hopefully this'll be the year I can retain some of that discipline after I'm done, instead of just going "Yay! Exhausted! I've earned the right not to write anything for a while!"