It's not uncommon to get a free game or two thrown in when you buy a new video card from Nvidia. It's currently offering a choice of Ghost Recon Wildlands or For Honor with a purchase of a GTX 1070 or 1080, for instance. But it recently came to light on NeoGAF that what you can actually do with those codes has changed. Instead of simply redeeming your freebie and getting a Steam key, you must now claim the game via the GeForce Experience software, and it will be tied to systems with a "qualifying GPU."
"Game coupon codes offered as part of a qualifying GPU or PC purchase are intended for use by the purchaser. As part of the coupon redemption process, Nvidia uses GeForce Experience to perform a hardware verification step to ensure the coupon code is redeemed on the system with the qualifying GPU," the coupon code redemption FAQ states. "Redeeming the code on a qualifying system helps Nvidia verify the game is redeemed by the purchaser."
As Engadget points out, this could be an effort to clamp down on people who buy a GPU, get the free game, and then refund the hardware, such as happened last year with the release of Gears of War 4. Which is perfectly fair—but it also prevents people from selling unwanted keys (possibly also fair, depending on where you fall on the subject of key resellers) or simply giving them away to a friend. And of course if you want the free game, you'll have to use GeForce Experience, which may be part of the goal as well.
Update: Nvidia confirmed in a statement that the new procedure was put into place earlier this week, alongside the rollout of the Ghost Recon Wildlands/For Honor bundle offer. "The terms and conditions of the game bundle require that it be granted to the purchaser of a qualifying GPU," a rep said. "This method helps us verify that the terms and conditions are being followed."