Look at it from the recipients' point of view. You are being given, not some random gift that you may not want, or even know what to do with...but the opportunity to get what you would really like and find useful. And if you are going to pay a little over and above the amount on the gift coupon, it's still a bargain for you.
From the point of view of the person giving the coupon, too, it's a good solution to the gifting problem. You may not, often, know the person/people concerned, or their tastes in books or things...so if you give them a gift coupon, it ensures that they will get what they want, and think kindly of you for allowing them to do so rather than juggle with gifts that they have no use for.
And of course...from the point of view of the retailer, it is the best of all. You are taking cash in advance of a possible purchase, which means that you have that cash in your coffers, working for you. When the person with the coupon comes to buy from your store, it is almost guaranteed that s/he will add to the money and get something (especially if you have raised your prices in the meantime!). And it's even better if the person cannot come in time to redeem the gift coupon....the coupon lapses, and the money is yours without having to sell a thing! I wonder if there is a reliable study made on how much stores make on gift coupons....it's very surprising to me that in India, not too many stores offer gift coupons, given how very advantageous it is for them.