Banners and such are PUSH for a customer, he didn't do any action to call the stuff in his eyesight. Coupons are just tingling at the side and if a curiosity makes a user to click on it, this generates PULL effect and he is more likely to engage with a coupon. Or in the other words, this means coupons are a holy grail of advertising :D

This is due to human curiosity which in turn generates a pull effect.