When you want to ride at a fast pace by yourself or with a group for a certain distance or a certain period of time, also when your going downhill and want to go at speed. So realisticly you wouldn't ride at the drops all the time because you would end up with a sore back. Which is not appealing. So most people would alternate from the hood to the top of the bar to the drops depending on the speed you want to maintain for a given time or distance.