Think back to trips to Disneyland or Disneyworld as a kid. Remember Disney Dollars? Not only were they fun and kid-friendly, creating a distance between real currency and purchases within the park...but they were also
redeemable only at Disney parks.
This is the concept behind all loyalty programs, in a nutshell. It's about giving ostensible rewards to loyal customers in an attempt to create what economists call "switching costs," i.e., the degree of hassle involved in moving from one service provider/vendor/business to another. If you've got enough points built up with one provider (e.g., American Airlines), you'll be reluctant to spend money on other providers (Delta, for instance).
Side benefits include increasing customer basket size, increasing purchase frequency, converting customers across categories, and increasing share of wallet per customer.
As loyalty programs go, Amazon Prime has been stellar. It's very costly for the company, and it had Wall Street in a frenzy when it first launched. But it works (and it's also very beneficial to consumers who use it).
I can't say I see the value in this program as much as I do with Amazon Prime, but the logic makes sense.