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.
On the other hand, you can also just host your own account in your own server, if you have the ability to do so. Sure not all servers federate with each other, but you can make sure that at least all servers federate with yours (if you’re a good netizen).