> You're building up a portfolio of writing about topics that interest you.

This reason resonates with me immensely.

You're not just writing about what you've figured out, sometimes you're actually deepening your understanding as you write! Writing is the thinking process: https://news.ycombinator.com/item?id=32628196

I have been writing every day at my blog for three years now, and it's been very rewarding for me to figure out what I actually care about and seeing patterns.

I like thinking about it like a bunch of skateboarders lugging the video camera around to capture the moment. (They did this before social media!)

P.S., You may also enjoy the similar sentiment in this post: https://news.ycombinator.com/item?id=42992159