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.
With that in mind, apple_enthusiast seems to have an order of magnitude more users than this.