People find it very hard to resist anything they can obtain for free.
According to Dan Ariely, a lot of irrational human behaviour can be put down to a hatred of the prospect of wasting money. He offered students a Hershey's Kiss for 1 cent or a (large, delicious) Lindt truffle for 15 cents and most of them went for the truffle (a much better bargain). But if the Hershey's Kiss was free and the Lindt truffle was 14 cents, most opted for the Kiss.
If something is free there is absolutely no risk of wasting money as result of making a bad decision and people heavily favour this option. The idea of getting something for free is extraordinarily motivating. This explains why 'buy one get one free' promotions are much more attractive than '50% off'.