Your ID on the server changes every time you login. You can find your current server ID by pressing ~ (tilde) and looking at the scoreboard.
Your ID is used partially for OOC (out of character) instances, and also for in-character instances where someone needs to wire transfer money to you. See "How do I initiate a wire / bank transfer to another player?".
Note: Do not confuse this "ID" with your "identification card / driver's license" item in game. They are completely unrelated.