This tactic leverages the revenue generating possibilities of third-party advertising without a high level of risk. The managers of this particular site are very focused on the fact that their site's whole purpose is to sell their product and they won't let anything get in the way of that. Not affiliate banners. Not third-party advertising. Not even their own VPs.
Nothing stands between their customers and their product.
However, these managers also recognize that their large volume of customers will certainly be interested in other related products. These managers know that in order to monetize their customer base to the fullest, they must look beyond their singular product sale. They don't want to expand the line of products they carry - they are doing quite well by focusing on their niche. They could accept third-party advertising for these other products, but that might distract customers from the primary mission. They are also negotiating referral commissions to be paid on all sales generated through this digital coupon book.