I write to let off steam.
Basically, I have issues left from my childhood and adolescence (and even further still) that I'm only just now realizing affects practically everything I do. So a lot of my writing (fanfic, mostly) tends to deal with some of those issues, as if by writing out what happened in a fictional setting helps me come to grips with what happened to me.Some of my readers have noticed a continuing theme of abuse and non-consensual rape in my stories; that's because those things happened, and writing about those events and the aftermath of them, helps me work out some of my own unresolved issues. By having a fictional character deal witht he aftermath of such events, i can find my own closure with what happened to me. It's an odd sort of therapy for me.
And it desn't hurt that my writing style seems to be popular with a lot of people, and the sympathy they feel towards my character(s) feels like it transfers to me. So I get the support and care that I didn't get when I was going through my own ordeals, and it helps. So yes, writing for me is therapy of a sort.