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.
I mostly agree with this. Here’s how I use the Fediverse:
I have an account on every instance that I want to browse, while keeping my own personal instance up in the first tab. I browse multiple instances, and when it comes time to comment, I just add the post to my own instance and comment away.
I use all the other accounts as lurker accounts. That way, my identity on Lemmy is consistent but I still get to browse anything I want without worrying about everything federating properly.