So that one entity doesn’t control the entire platform, mostly. With different instances, it’s difficult or impossible for a rich jerk or corporate ownership to make unilateral decisions that make the platform unusable. That could happen to any instance, sure, but there’s a lot less friction migrating to a different Lemmy instance than finding a new app entirely.
It also allows different people to join instances with policies that match their preferences. If I want porn I can go to a porn-friendly instance that won’t ban NSFW content; if I want piracy I can go to an instance that allows that; but neither of those would threaten a different instance who wants none of those things, and can freely defederate from the porn and piracy instance.
Why do we need separate instances
So that one entity doesn’t control the entire platform, mostly. With different instances, it’s difficult or impossible for a rich jerk or corporate ownership to make unilateral decisions that make the platform unusable. That could happen to any instance, sure, but there’s a lot less friction migrating to a different Lemmy instance than finding a new app entirely.
It also allows different people to join instances with policies that match their preferences. If I want porn I can go to a porn-friendly instance that won’t ban NSFW content; if I want piracy I can go to an instance that allows that; but neither of those would threaten a different instance who wants none of those things, and can freely defederate from the porn and piracy instance.