Posting on other instances?

I like that Lemmy can post on other instances (communities on other instances), but thinking about it, would that not present a potential issue with centralizing information about a topic on one instance?

I know it's kind of similar to having themed Mastodon instances (and I also don't really understand those), but it seems like a potential issue?

I know multiple instances can have the same communities, but that then seems like fracturing the conversation?

Could this just be due to my limited knowledge/use of the fediverse?