You know, the
ad in the
newspaper saying get $1.00US off a pack of
hamburgers. The main purpose is to spur
sales of a certain
product, as well as get you to
shop for other items while you're in the
store.
What's strange is that they have expiration dates. Why? Simple, because it motivates you to buy it now, not later. Some items are only on sale for a limited time. Also, it's to reduce liability.
Only a small percentage of coupons are redeemed, though literally Billions are printed. There is a small chance that somehow all the coupons will be redeemed at once, bankrupting the company instantly. As an insurance measure, the expiration date makes it less of a threat, as there would be no obligation to give the people their money.
Unofficially (Confidentially?), most stores will accept the coupon, even if it's a little late, if you have a proof of purchase, but don't expect it to always work. I lost my $30US rebate from HP due to a day late.
The real question is why so many say in small print "Actual cash value 1/20 of a cent."