inb4 sorry, wall of text. [Also, note: this is my personal take.]
I think that communities are not enough because of a few things.
One of them is that most people expect to be able to behave the same way, abide to the same rules, and contribute the exact same way, to have the same level as expertise, as long as they're in the same site. Sometimes this is not of the best interests of the group.
You see this a lot in Reddit, and I believe to be one of the main factors behind large subreddits going downhill, no matter the best efforts of their mod teams. r/linguistics is a good example of that - the sub was supposed to be for people at least familiar with Linguistics, sharing information, but most posts and comments there nowadays are from laymen assuming shit out of nowhere, because that's how your average redditor behaves.
You could follow the local rules, sure; but most people will not. Unless there's a higher degree of separation between both group.
Across instances however it's clearer that you're in a different site, with different rules, that you may need to behave in a different way. And that a contribution that may be welcome in one instance might be harmful for another.
Another is that different instances host different demographics. My example is too mild to show it, but imagine the following two instances instead:
- charlielemmy - intended as a safe space for trans people. Mostly so they can chill together, shitpost together, nothing too serious.
- danlemmy - intended for people who have a really dark sense of humour, and who'd gladly poke fun at everyone, including trans people. Definitively unsafe.
If you're the admin of one of the instances you definitively do not want people from the other instance. In charlielemmy this would make the environment unsafe for its target userbase; for danlemmy it means that you're going to get people pissed at the dark jokes, and potentially starting arguments.
after all, a user could want to participate in both
Federation makes it easier to handle: register in both alicelemmy and boblemmy.