@zaitcev @waltercool @mrsaturday Convenience for the end-user, I guess. Why have your customers/clients jump through hoops creating another account when they can use something that they already use?
Also, I've seen cases where game developers create forums for their games, but some people will refuse to use said forums because "they're outdated", so fans will create unofficial Discord servers instead and they become the popular place to discuss said games.
@frost @waltercool @mrsaturday @zaitcev I think there are some add-ons for some popular forum software that enabled ActivityPub support. I know Discourse plans to include it, if they haven't already.