The point is flexibility and no lock-in. If you buy into the Twitter platform and eventually you're not happy with the way it goes, moving away has a huge cost. If a federated architecture peers can come and go relatively easily.
See email for instance: if you're unhappy with your current provider you can move to a different one or even roll out a new server and you can still interact with the other users.
Simply interacting, yes, but email and messaging cannot replace Twitter. What makes Mastodon viable is the identity, multicast and backlog of toots(tweets?), which does not transfer as easily between instances.
See email for instance: if you're unhappy with your current provider you can move to a different one or even roll out a new server and you can still interact with the other users.