@everetra - Yes, there have been a lot studies about the health benefits of expressive writing. Some of these studies have focused on college students. In one expressive writing program the students were asked to write about their feelings about coming to college. It was found that these college students (the ones who participated in expressive writing) had better long-term memory and fewer visits to the doctor.
If you ask me, I think it has to do with the relationship between stress and overall health. People who “bottle it all in” get sicker more often, while people who open up feel better. I don’t think it’s more complicated than that, but expressive writing takes this very simple principle to the written page.