Having accounts in different Mastodon instances, for example, allows you to see more of Mastodon, because not all servers federate with the other servers, and some servers will recommend you different accounts, or will only partially show you posts from accounts on other servers. It also allows you to test out each one of those instances, allowing you to have some time to think before deleting or migrating your accounts. Such an approach allows you to have a different experience on each instance or server, so that eventually you will have different servers for different kinds of content.
It takes a bit to get used to it IMO. Whenever I’m browsing another instance, it’s a bit awkward having to copy the link of whatever post I want to share, then open my instance, then paste the link into the search bar, and finally actually press the share button. In addition to that, a few Mastodon instances are “private” and won’t let you browse them unless you have an account in them, which feels off considering I technically already do have a Mastodon account lol.
I also found Lemmy to be incredibly janky so far whenever you’re looking for content from other instances.
Really I think these things will be eventually sorted out as the Fediverse matures, but part of that maturing process will probably be having a way of uniting accounts on multiple instances or maybe even multiple platforms. After all, as long as this is just an option, people who don’t like it will still be able to have separate accounts on separate instances.