What happened two years ago?
DoW converted from a "pay as you go" system of access to their games to a perpetual rights access.
Originally, you could use the codes to buy 6 month blocks of access to all of their games. When they converted, you could use the codes to buy permanent access to specific games. The more games you wanted, the more codes you needed. Obviously, you could also supplement codes with cash.
Codes over and above what were necessary to buy full access to all of their games could be converted to discount coupons in their online store.