Diaries, blogging, and other forms of personal documentation are a way to measure progress a bit more objectively than our imperfect memories can. Many, if not most, people, quickly forget the good things they've done. Documentation is a way to overcome this natural memory loss.
As with all knowledge, it's usually best to record it in a format that supports efficient retrieval. So, it's good to be brief. Just a few lines.
Learn and Earn is a Steem Bounty project that pays steem to people to document what they've learned, briefly.
I think it's a fantastic idea, whether you get paid for it or not.