The nature of federation means that even if you never post anything to my instance, your garbage may still be visible to my people via tangential interactions on shared linked instances. Bans solve this on the local instance level and are by far the easiest way to accomplish this.
(To be clear, the “you” in this is not referring to you specifically OP, it’s a general term)
It definitely still caught me by surprise. Not like it might have in 1996 - I just played Baldur’s Gate 3 within the past year, after all - but the first time I played I definitely managed to get almost every single piece of bad evidence against me that I could rack up without even meaning to.