Mastodon: @[email protected]
I have seen others explain it like email. Email accounts will still exist regardless of if, say, Gmail shuts down. Same with fediverse instances. Unlike traditional social medias like Facebook and Twitter, people can talk to people on other sites. Imagine if people on Twitter and Facebook could reply to each other
The only exception is if one of the two servers has chosen not to federate with the other. In that case, you would want one on each account. However, that’s pretty rare as of right now.