I started, to get my thoughts recorded in case I died young, and my kids wanted to know who their Dad was. They're older now, and know me well.
I also got most of the big stuff I wanted to say, down. I'm working on one at the moment, but its a big 'un. I still upvote and comment, but I'm a reasonably big account, so I'd rather somebody else get the posting rewards (unless I have something I really want to say).