I believe the point of the code is that if you decide to purchase the game, the person that referred you got credit for the referral. WoW used to give like a free month for every sign up you brought in. I couldn't tell you if they still do any of that though.
Not sure these are even needed anymore, but I got some in my Overwatch box and I don't need them. Please post if you use one of them. Not sure what the dude above me is saying, I don't get any kind of credit for posting these as a referral or anything AFAIK.