I started so it would help me be more organized and also as an extension of my brain. i have all this stuff floating in my head and i need to write it down so that it doesn't clog my brain anymore. i have a personal one and also a work one.
for my personal one i've started recording my struggles and successes with overcoming my anxiety (it's been really bad the last few months). Writing it out feels like therapy. I can also track better over time how frequently it's a problem.
I'm also tracking habits as well such as exercise, working on PaperDemon, etc. I have a goal to spend at least 20 minutes a day on PaperDemon, either commenting on work, or working on developing the new version.
The bullet journal is also helping me set goals and make sure i don't take on too many things each day. I can easily plan to do a task in the future and not have to worry about it today. It's probably one of the best self improvement things I've learned.