I'm feeling lazy, so heres the wiki explanation...

At high engine speeds, an engine requires large amounts of air. However, the intake valves may close before all the air has been given a chance to flow in, reducing performance. On the other hand, if the cam keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions. For this reason, pure racing engines cannot idle at the low speeds (around 800rpm) expected of a road car, and idle speeds of 2000 rpm are not unusual.

Variable valve timing gives you the best of both worlds, rather than valve timing that just works across the whole range.

 

I go there when I have to for parts. They always treat me right. The place is spotless. Each time, I take a trip to the showroom, and around the lot drooling on all the cars. They also have an aftermarket facility near their race shop where they display Champion ALMS Audis. Really cool.