I write here. I used to blog, but I decided I don't really like publishing my personal things to the public. My compromise is permies, where I publish garden and project things but not so much personal things.
I tried to write fiction for about ten years. I still occasionally write a story, but now only when I feel the urge and never because I think I should be writing. I did the nanowrimo thing for a while. I really wanted to tell great, epic stories, but I realized I don't really have the knack for compelling storytelling. Before that I wrote poetry. Some of it was alright, most of it was crap. I still write poetry when something really traumatic happens in my life. It helped me through the suicides of several friends.
Now, other than on permies, I just write for myself. I journal about my garden and the weather and things I've done and want to do. Looking back through my journals has proved useful for gardening, I can see when I planted something, and if I might adjust that for the following year, or about when would be good to harvest. I can also reflect on how the weather probably influenced my harvests and productivity. It also helps, as others have said, to just get things out of my head and onto paper.
I also have three professional books in the works. Hopefully I will publish them one day. They're all on education or language topics. If I don't get them published, oh well. Writing them helps me organize my thoughts around my day job.