Another reason for the clamp below the stem relates for racing. Many people, including myself, prefer to keep our stems "race tight" meaning tight enough to control the bike, but loose enough to slip and allow the bar to turn in a crash. The reason is that the slippage can save a fork from twisting when the wheel spins until the bar hits the frame.

Some people find that race tight stems slip a bit and lose the headset adjustments on bumpy roads, so the clamp below keeps the headset tight, without giving up the race tight approach.