In an article from 2012 Rajkumar Venkatesan and Paul Farris looked into the economics of coupons:

• In 2010, 3.3 billion coupons were redeemed, saving people $3.7 billion.
• These massive numbers only represent 1% of all coupons printed. 99% are never used.
• But that 99% is valuable. People who received coupons but didn’t use them still increased their purchases of the brand.
• Those who use coupons are only responsible for 40% of an increase in sales after a coupon campaign. 60% of the increase in sales come from the 99% who don’t use a coupon.
• Even if they’re not used, coupons help boost awareness of a brand, and serve as little marketing leaflets.
• Redemption rates still matter. A coupon which is used by few people might be a sign of a badly designed coupon.
• This is good news for companies like Groupon who have been criticized for not generating sustained business growth for companies.

Read more on the Harvard Business Review.