As Ricky Bobby says, "I want to go fast." If you've ever seen a race car draft behind another race car, you know the one behind has to put out less energy to maintain the same speed as the one in front.  The one in front is "breaking the wind." It is the same concept with bicycling.  Sitting in the back, takes a lot less energy than sitting on the front.  Riders will try to stay in the back so they have energy for the sprint at the end!