CD keys are useful for the following (but not limited to):
-Download the game from a torrent and then use the CDkey to activate it. -Buy the actual game from EB instead of using their CDkey use yours and then return the game for cost price.(if you can still do that)
That's all I can think of right now.
Note: This may work for some games and maybe not Call Of Duty. I don't know the specifics.