Imagine that the main instance starts to blocklist words and censor content. Or ads. Having a way to move all your history, follower and follow list would be great so deter such issues since users can abandon your server quickly and you would just lose the userbase you were trying to monetise.

Imagine I decide to invest in a private server and want to create my own instances and move my Lemmy and Mastodon accounts there to have better control of what happens.

Having to resubscribe to everything would suck and specially in mastodon, having to generate a new userbase from zero would just deter users from such things.

Another example, suddenly the Mali govt decides to close lemmy.ml, that would suck. Having a way to changing the instance would be just great.