Maybe he wants the nearly 3,200 blue coins or maybe he has another stackable discount or enough blue coins to buy the entire bundle. In the end, his portion will be $15.91, $0.92 cents more then the Steam one, but he is getting over $3 worth of blue coins. Not too bad, really. It is essentially getting the game for $13.
The blue coins are always nice on GG, but only when it makes sense. I hadn't seen the 4 pack on Steam, don't know how I missed it, but it did seem odd to me. Do four packs not come up when you search Steam? Do you have to specifically click on an individual game to see the four pack option? Now that I know they have 4 pack for cheaper on Steam the GG four pack is not a deal at all.