How does having an account per onstance work?

For context i did not know you can use an account from a single instance to post on another. With this in mind I have a few questions:

1. If I can use one instance account everywhere, why can I make multiple accounts?
2. Why isn't there a centralised account system?
3. What's the use of having per instance accounts?

I'm currently considering keeping this main account and deleting the ones on other instances, because if i can interact with those instances I don't see the need for an account.