If you own Serious Sam 3: BFE, you may notice you've received a discount coupon for Duke Nukem 3D: Megaton Edition. Conversely, if you own Megaton Edition, you've received a discount coupon for BFE.
Devolver Digital is thrusting codes upon the Steam accounts of its customers as a "thank you" and to encourage a few sales, because for some strange reason it thinks rewarding customers, rather than punishing them, is a plan for success. Such unusual behavior in the game industry.
If this plan goes well, it will be the first of several such promotions. Cool!
Making it Rain (with Steam Coupons) [Devolver Digital]