Why there is $address->hasCouponCode() in qoute's _validateCouponCode() function?

Question

I'm just courius why there is check for $address->hasCouponCode() in _validateCouponCode (Mage_Sales_Model_Quote).

I think that there is no such data in model nor in database like coupon code for address.

I'm asking because our developers are overriding this method, and they are checking this condition but always $address->hasCouponCode() returns nothing.