Seat further back stretches out the torso, forcing you to reach further. Reaching further flattens out the body, lowering it. This improves aerodynamics and helps with breathing. It also seems to even out the amount of weight on both wheels, rather than a more upright position which tends to put all the rider's weight onto the rear wheel.

The more weight you can get onto your front wheel, the faster you will be. See Graham Obree: