|Image by SimonDoggett / Flickr / CC-BY|
Once you understand it, you realize how it is used in advertising.
Consider the buy one, get one free offer of Domino's Pizza (India) sent by SMS text on mobile:
"Double the Indulgence for ur tastebuds with Dominos. Buy One Pizza & Get One Free. Order Online/Mobile @dominos.co.in Cpn: CRM1328AADE6 Vld:29May.T&C"