See also 'rubberducking' and previous discussions of this on LW. My basic theory is that reasoning was developed for adversarial purposes, and by rubberducking you are essentially roleplaying as an 'adversary' which triggers deeper processing (if we ever get brain imaging of system I vs system II thinking, I'd expect that adversarial thinking triggers system II more compared to 'normal' self-centered thinking).
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.