Gaming.
Shadow Banning is very useful for spam bots. If you let them know they’re banned, they’ll just open a new account. But if YouTube keeps accepting their comments with a smile on its face before immediately tossing those comments into the shredder, it’ll take some time before the bot figures out what’s going on.
In my humble opinion, a twitter-like platform needs a big central algorithm that can associate posts with certain topics and interests to be able to serve up an interesting feed, because most people are just kind of shouting into the void and that endless storm of posts has to be filtered and organized somehow, otherwise everything you see is just benign uninteresting garbage. Lemmy/Kbin have the advantage that by nature all posts are neatly sorted into topic-based communities, and it’s a lot easier to subscribe to the stuff you find interesting, and block the stuff you don’t like.
I think it should be more akin to something like email. There is no one entity that controls all emails. It’s lots of independant servers and clients able to communicate with each other.