It is surprisingly effective at storing not just what I did with my day, but random thoughts and ideas, solutions to bugs, etc. All I have to do is type something every day.
In 2 years I have typed around 150k words. For reading he whole thing is just one plain html page, searchable in the browser.
I'm currently experimenting with documenting my reading, while the book is being read (instead of a summary at the end). We typically forget what we expect while we read, and only remember the final impression. Taking notes at the end of each reading session, not just about what I just read, but also about what I think will happen next, and revisiting that later, is surprisingly interesting.