People often reveal more about themselves then they intend to. Most of the time, it's not a big deal. If you piss off the wrong crowd though, they will hunt through your posts looking for every scrap of info so they can dox you. People want to be able to delete old posts when they realized that talking about which neighborhood they live in, which month they were born, and where they go to school might not have been a good idea.
Reddit is centralized, so unless it was archived there's a good chance your comments will disappear. Stuff in the fediverse is a lot more likely to stick around somewhere.