I think the code is for so that they can narrow it down and then search for your name in their mailing list from all recipients in that code mailing list. It's only a guess.
If they were going to do this, my guess is that they would probably scan all of the lists (instead of a code-specific one), just in case someone is supposed to be receiving this offer legitimately enters the wrong code or something. I'm thinking this code is more for marketing reasons, to see where you are seeing the ad or the reference to the offer from (like they always ask you when you order from a catalog, for example).