This is what I found in the comments…It’s for the people who pre-ordered.
"This was a Pre-order Bonus at selected retailers.
Quote from official page:
Exclusive Pre-order Incentives at selected retailers.
Available at selected retailers, players who preorder the game will receive an exclusive Steelbook, and/or the Opel Kadett C GT/E"
But I agree. That’s pretty crappy.