You are purchasing a AMD reward coupon and not the key directly itself. Activation instructions are listed on the link above or thedirect link is here
Bonus games to choose from include:
Hitman Absolution, Sleeping Dogs, Deus Ex - Human Revolution, DiRT 3
As a general guide I have seen all these bonus games go as low as US$5 each. If any of these games interest you it makes Thief approx $10 cheaper then anyone else. If non interest you its still a lot cheaper then anyone else.
Keep in mind Dirt 3 might be in a bundle soon but there has been no firm confirmation of this
I already own all those bonus games :(