So that you don't need to seek out "which instance isn't federated with [instance I would wish to defederate from had I the ability]" and can instead simply click "block instance." And could do this without hosting my own whole ass instance.
Say another NSFW instance pops up and gets really popular...but it's only furry and futa, and a lot of it, and new communities all the time pop up for vore, and guro, etc. Sure, one can whack-a-community everytime one pops up if they don't want that being prominent in their all tab, or they can just click "block instance" and keep it pushing. It isn't something bad enough I feel I could (nor should) bother my instance admin about defederating, and if he disagreed with me I'd like to not have to create a whole new ass acct just to not see c/specificguro pop up on my shit after banning c/guro and c/leftguro, and I don't want to block all NSFW because A) It isn't always porn, that tag is sometimes used erroneously, and B) I'm cool with some titty, but cockvore is a little much.
Tbh how do you not see the need for this?