most of the times, the shops earn money with that, but in the end you don't really earn anything. For exemple, if you buy a product that lasts 1 month, and it costs you 1$. Another product is 60 cents, and it lasts 2 weeks. They will offer you 1 couppon to the 70 cents product, because they wouldn't sell that product anyways. And also sometimes they give you 2, but you cannot use them at the same time (with a 10 cents discount for exemple), wich meant you probably just spent more, because instead of buying the excelent product for 1$ that lasted 1 month, you are paying 1.20 for two lower quality products.
Generally that happens with products that are a little more "costy".