Federation is good if you want to stay within a community but also have a chance to interact with others.

I.e. you mostly care about technology foo but occasion delve into epic poetry, and it's nice to interact with both footech.social and epicpoems.read. Also, being able to consume personal publishing (blogs!) from within the same app is quite nice.

Why not several identities, and aggregation in the client? You seem to need to be in the community to fully interact with it on Mastodon, anyway.

The steelman for federation is that email survived the rise of the big platforms despite no-one owning email, so making other applications follow the email model means they too could be free from central ownership.

So did forums, even with no federation.

You'd have to have a lot of trust that the one instance wouldn't get enshittified and end up as another X / Truth Social / BlueSky.

Different instances can also have different rules, different moderation and different federation.

Edit: and exist in different legal jurisdictions, and also be harder to ban or regulate.

Having many instances is not the same as federation.

Isn't one a subset of the other? A system with federation must have multiple instances, but a system with multiple instances doesn't need to federate (in the sense of information passing between independently managed instances.)

Yes. My original question was whether federation is necessary for the kind of communities Mastodon serves, not whether the web must have multiple websites.