If a border guard is on Earth, then he protects his country—right at the very first line. And if he is in another world? Does he protect the peaceful life of that world? Of course! But to a much greater extent, he protects his own world—his Earth.
And the profession is chosen in a rather strange way. For someone to move into a different world—the central one for all inhabited worlds—they need to do some exotic gymnastics. For someone else—to read a tear-jerker novel. And for someone else—after taking a bath, to make just one step. And that step will change your entire future life. And it doesn’t matter who you used to be. The only thing that matters is who you become.