I've got a diary that goes back as far as 1991. I thought I was doing it just so that I'd have a record of my life to look back on (for some reason I like to have a handle on my history), but a lot of it is so woffly that I can hardly bear to read it, though the digital volumes can at least be searched, and some time ago I went to the trouble of summarising the paper volumes in a digital document, so whenever I want to know when this or that happened, I can usually find out without too much trouble. I rather regret throwing away my pre-1991 diaries.
So my motive for writing a diary seems to be mainly down to an interest in (my own) history, and the document has been of practical use when I've needed to know when this or that happened, but I think there's a therapeutic element in the actual process of writing it as well. I like to feel that I've spat out my experiences.
I also seem to get something out of writing posts for WP, regardless of whether or not anybody turns out to be interested in what I've written (though that matters to me also). It helps me crystallise my thoughts on topics - sometimes they're about things I've not thought about much before.
In his later years my father turned to writing about his special interest, and sent the results as articles to one or two American magazines that were about that interest. His work was well-received and he made a few good friends that way. He told me that he felt he'd become a big fish in a small pond. Certainly writing those things was good for him. As he was (very likely) on the spectrum, he barely had any "real-life" friends, so it was a good social outlet for him. After his death, one of the guys who had liked his stuff set up a website with all the articles on it, which AFAIK is still there.
A long time ago I began to co-write a fiction book with a friend, and that was great fun while it lasted, though the project fell by the wayside when I realised that it would be too hard for me to complete it. Is fun a kind of therapy? Certainly I feel happier when I'm having fun (to state the obvious), and I suppose the main point of therapy is to become happier. Maybe it's better, though, not to medicalise it (isn't therapy what you do when you're emotionally unwell?), but instead just to call it an enjoyable pastime.
Anyway, whatever you want to call it, I'd say it's a good idea to try writing and see if it does you any good.