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’s just so many websites to log into. I would really to see a fediverse app that unified timelines from multiple accounts.
Not sure if I understood things correctly as I only use one, but Liftoff seems to have this functionality. I didn’t care enough to test, but maybe you can try