Keep back-ups (and backs-ups of the back-ups) for just about anything I ever write. I figure, I never know what I may want or need to do with all, or part, of anything I've written. I may never go back to something I've written, but it's there (and with dates) if/when I ever need to for any reason. I save some of my own, longer, forum posts "just in case". I even save some of my HP-Answers replies if I think it makes sense for individual reply. It's so "nothing" to just keep back-ups/copies of anything we post online, I can't imagine why anyone wouldn't do it. (I don't mean for social sites, but I don't spend time on them. I mean for writing, or "writing-type", sites.)
Here's one reason I keep copies of things like forum posts/answers: People can come on here, ask a question, get a bunch of answers, and use the best answers on another site (maybe one where people get paid for having the best answer). Whether I, or anyone else, bothers following up and checking on that kind of thing depends on the circumstances and particular thing that was written; but keeping copies for reference, follow-up, record-keeping, etc. etc. just isn't a bad idea. You never know when you may wish you had kept copies of something for one reason or another.