Lemme give you 2 scenarios to explain why defederation is an important option:
An instance with views and forms of diacourse wholly incompatible with yours. As a hyperbolic silly example to illustrate, let's say there's a community of cultists who believe in Cthulhu. They encourage a steady diet of kidnapped babies, and ritual drownings. They brigade like crazy, and their moderators encourage them to do this. Anybody who doesn't follow the great old one is an idiot, an asshole, and they express this in extremely vulgar terms. If they were a reddit community, they'd get banned. Here, they get isolated to their own little corner where they can't scream obscenities at people.
An instance with thousands of bots spamming out innocuous looking links that lead to malware. Again, if they were a reddit bot farm, (hopefully) they'd be banned. Here, as it's an open source project and you can't restrict who uses it and for what, defederation is the best you can do.
Lemmygrad seems to be a bit of a target of defederation by many instances, and there's probably some history there that I'm not aware of as I'm also new to Lemmy. Even though I may not agree with some of their hardline views, the users seem to be respectful when commenting over here on lemmy.ml, and the very few times I've commented over there, they've been cool. Consequently, I'm glad lemmy.ml federates with them.
EDIT: I should probably state that even though I'm cool talking with the respectful users, had to block a few communities there. I don't want to see the Death to NATO community cheerleading the Russian invasion, for example.