Seth Godin writes:
Coupons are a surprisingly subtle invention. Now that anyone can offer them (because now anyone can have a store), it’s worth a second to think about what they’re for.
First benefit to the marketer is that coupons allow you to offer different prices to different people.
There’s a reason that most coupons are not trivially easy to find or redeem. By trading effort for a discount, the marketer says, “if you care about price, I’ll sell it to you cheaper, but you have to prove it.” Hence the original idea behind Priceline. It was intentionally awkward to use so that the airlines could be confident that only the fare-obsessed would use it.