It's a way to make my thinking more concrete without diving all the way into the complexities of the code right away. So sometimes, what I write down is design documentation, and sometimes it's notes on a bug report, but if what I'm working on is user-visible, I start by writing down the end user documentation.
Writing things down lets me interact with them as words on a page, which are more concrete than muddled thoughts in the head, and much easier to edit and reason about. Code constrains to existing structures; a blank page frees you to explore and build up new ideas. It's the essay writing process, applied to software development, with a side effect of making sure everything is documented.
Also, end-user documentation is best when it doesn't assume that the user has any prior knowledge. The point in time when I'm closest to perfect lack of knowledge about something is before I've built it. So, that's the best time to document it.
I understand what I'm trying to tell you better now that I've written it down than I did when I started. Hopefully you do too.
 I'll often write a bug report down even if I have found the bug myself and am going to fix it myself on the same day. (example) This is one place where it's nice to have bug reports as files in the same repository as the code, so that the bug report can be included in the commit fixing it. Often the bug report has lots of details that don't need to go into the commit message, but explain more about my evolving thinking about a problem.
 Technically I'm even more clueless ten years later when I've totally forgotten whatever, but it's not practical to wait. ;-)