I just write whatever i feel like. It's usually just a running commentary on my day. It's not meant to be of interest to anyone, not even to me. I just find the act of writing is comforting in the same way that couples ask each other about their day. I'm single so i don't have anyone to do that with and i find writing fills this void nicely. Perhaps it will be interesting to read when i am old. Or perhaps not.
I've been writing daily for twenty years. I write thru the day, every few hours, about a meg a year. That's a lot of writing!
Perhaps a university psych student could get a thesis out of it, on the thought processes of a bipolar. But if not, writing to comfort myself is still reason enough.
On the rare occasions that i have over-the-top emotions it's helpful to spend a few hours typing out my feelings. It helps burn them out. On the whole tho, it's not very interesting material: "I had a nice poop."