If you go back enough years, the legacies would sell last-minute seats at deep discounts on the theory that something was better than nothing. So a pricing incentive would fill seats that would otherwise take off empty. This made last-minute a favorite of students and impulse fliers.
When the airlines figured out that most of the last-minute buyers were business people who had no choice but to go right as way, as pointed out in some previous posts, then the pricing model flipped.