Professor William Morgan's distraction hypothesis sounds plausible -- that the distraction takes people's minds off of worrying. Along those lines, productive activities in general may have positive effects not just for their distraction properties but for their intrinsic ability to contribute to a sense of accomplishment.

The distraction hypothesis probably doesn't sit well with those who argue that depression has a physiological basis and must be addressed medically. It's not clear to me whether one side is right and the other wrong or if it's a difference of degree. It seems to parallel ADHD treatment issues, at least on the surface.