Rewards programs repay loyalty with additional savings. To be eligible, you often have to buy more in order to save more. This sounds easy, and it can be, but you won't always find the best deal on all the goods you want -- even goods within a specific category -- in one place. Like grocery shopping, you're likely to find great bargains on a few different items distributed among the major retailers.
In a brick and mortar shop, obvious deals are designed to attract shoppers who will stick around to shop for other items they need which provide a better profit for the merchant. Because Internet shopping is less labor intensive than hopping in the car and driving all over town, it's much easier to suss out good bargains wherever they're hiding. Comparison shopping engines like PriceGrabber are dedicated to showing you the location of the most competitive bargains.
To keep you on their sites, or convince you that coming back regularly is a good idea, online merchants have to do more than lure you in with one or two great deals. Shopping rewards are an effective marketing strategy because they encourage shoppers to accumulate purchases to reach different plateaus that will provide better discounts (or other goodies). This keeps people coming back or encourages them to buy more during a single visit.