Your game ID is how we confirm we are in the right account before we spend anything.
When our agent signs in, the first thing they do is check that the ID on screen matches the ID on your order. If it does not match, we stop and come back to you rather than buying into the wrong account. Login details alone are not enough for that check, since one email can be linked to more than one account.
It matters most if you play several accounts in the same game. The ID is what makes the difference visible to us, not just to you.
You can find your game ID in your in-game profile or account screen. If you are not sure where, send us a screenshot in the app and we will point it out.