I'm very new too but my understanding is that each instance can have it's own specific content. But, you can see all content from any instance if you sort by "all." If you sort by "local" then you will only get posts from the instance that you're logged in to. If you go to subscribed then you'll just get stuff you've subscribed to in that instance. Could be wrong tho lol

This link will help explain migrating from Reddit and what instances are and how they work independently and simultaneously at the same time. You don't have to create an account for every instance. You can follow another instance as a guest, you just won't be able to interact with any comments or upvoting/downvoting. Think of them as separate websites under the same platform that can all coexist independently but still interact with each other. But this helped me out a bunch! https://github.com/amirzaidi/lemmy

Hi, sorry for the late response. You don't need an account on another instance unless you want to interact with people on that instance and you want to use that specific account. I believe you can also use another account on a seperate instance as long as the two instances are federated with each other.

People tend to have multiple accounts on multiple instances because some instances arent federated with each other and thus you don't get the full experience. Take for example Beehaw and lemmy.world. If you have an account with Beehaw the only way to see lemmy.world posts is to have a lemmy.world account or an account on an instance that is federated with world or vice versa.

Another more obvious reason is that since instances are generally privately hosted servers it'd be wiser to have accounts on multiple different instances as opposed to one because things happen and server outages are possible.