I don't know if there is a unifying theory. Not one that works for everyone.
I came looking for something to help me get healthier, not specifically for weight loss or fitness. In the process, I have continued to get healthy, lose some weight and become fitter again. Here I get a group of people who "get" how I eat, which I do not have in the non-electronic world. I also get great pointers to good resources and other like minded blogs I am also following. All in all, a better place to spend my computer time than the casual games sites I used to visit.