It would be useful to be able to automatically subscribe to all communities that another one of your accounts is subscribed to.
The use case would be, a user originally joined lemmy.world, but the instance is overloaded and the user wishes to migrate to another instance. They could create a new Lemmy account on another instance and press a button to automatically copy all of their subscriptions from their original account to this new account.
It would save the user a tremendous amount of time hunting down all of the communities they were originally subscribed to and honestly make migration nearly painless.
This is kind of a weird advanced request, but I see it having a lot of value with the way Lemmy is setup.