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.
To each their own, but strong disagree from me. I’m just going to see 95% of the same stuff on each account.
I never did keep alt accounts on any social media platform. And if something is defederarated, then it probably wasn’t worth my time anyways.