The biggest advantage of clipless pedals is that your body gets used to pedaling with your foot attached to the pedal in the excact same position. Guys that ride clipless can tell you that after months (or years) of cycling with your cleat setup at 'X' position even a slight movement of 1mm off this setting will make your leg muscles hurt. Even the slightest change in position here will work different muscles or the same muscles in a different way. Some will say clipless allows you to "pull up" but studies have shown that cyclists dont put out much force this way. It does however give you the ability to eliminate dead spots in the stroke by pulling forward at the top or pulling back at the bottom. Think wiping mud off your shoes if you will.

Real reasons that clipless pedals are more efficient.