The Purpose of Writing

You can write for two reasons:

• to remember stuff
• to understand stuff

I believe the second reason is more potent that the first one because it helps achieve the first one with the added bonus that how you'll write will reflect how you think, and help you figure things out and solve problems when the first won't.

I also believe it's a great form of exercise for the psyche to embrace stream of consciousness writing, or something close, rather than spending time reformulating or looking for the perfect sentence each time you want to write something down, especially inside your own private notes / journal.
It lets you embrace and separate each mode of writing: the creation part, and the editing part.

Writing things down to remember them is akin to being an archivist, a librarian, you just store knowledge but you don't really use it.

Writing things to process your understanding of them, on the other hand, elevates your ability to reason and gives you a better understanding of the subject you're writing about. It helps you harness it, and use it to solve problems. Or take action on it. Or explain it. You're doing something with it, instead of just storing for the sake of storing.