You don't understand it right then.
Let's say you paid 10$ on RU, and spent half of the credits you got. Then you see a NA server is gonna open, and want to switch over. You can take the 5$ worth of credits left and transfer them to NA, which is a good service for newcomers who only heard about NA server recently and still have credits on RU.
It's also keeping people who don't swipe at all from being able to transfer over credits they worked for on RU.