Take the example of happy hour in restaurants: by offering preferential rates during a given period, customers are encouraged to come more often and to invite their friends. Similarly, the 1 bought = 1 free scheme in supermarkets is a simple and effective way of building customer loyalty by giving them the impression that they're getting a good deal.
It's a golden rule of marketing: to build customer loyalty, you have to offer them benefits. In exchange, they'll be more loyal to you, and more likely to tell others about you. Yes, but is it expensive? No. If these offers are part of your strategy, then consider them an acquisition cost.
Try to win 1 year's free subscription by playing our own game: "We're making offers too".