Yeah, but imagine that you are not bounded to server. You can just sail of fly to another island/server.
Typical you can play at your server but imagine that:
you meet someone on forum, and he is from another server. You want play with him but you won't lose your character, so solution is simply sailing to his server. It will allow to transfer between servers and it's realistic from point of view inside the game, it's not just magic switch to another world.
I can understand that, but someone could join a server with instacraft or something and take those benefits to a server not meant to have such privileges.