I like to think of instances as countries. Each instance has its own rules and culture, and that affects how the communities are moderated. For example, the beehaw.org instance is heavily moderated compared to lemmy.world. Continuing with our example, the Technology community on beehaw.org will be moderated differently than the Technlology community on Lemmy.world.
So each instance has its own reason for existing. Another example will be lemmynsfw.com which is an 18+ instance. Basically, when you register in an instance, you become a citizen of that country.
So in summary, each instance has its own rules, its own policies, its own culture, and the moderators of the community that is that instance are bound by them. So [email protected] is not the same as [email protected].