@haikushack, the idea is that if you migrate from one server to another, you're shutting down your account on the original server. If you choose to have multiple accounts on different servers (which is a valid option), then you may want to use a different app than the official app. Toot! on iOS for example allows multiple accounts and you can switch between them pretty darn easily.
@iiogama Thank you! I fully understand now. Everyone has been so kind to explain!