unless one just feeds you tons of ads and harvests user data. That's one reason why Gab, which is a fork of Mastodon, was defederated from most of the 'verse before Gab just went ahead and turned federation off.

You could create a Lemmy instance that made it far less user friendly to connect to other communities, and "forced" other users to join its communities because 'that's where everyone is'. That's one of the reasons why there is so much fuss over how to handle threads.net when/if they turn on federation.