On the Silvertop and Blacktop 4AG's, the VVT system adds 30 crankshaft degrees of intake cam timing (15 degrees of physical movement on the camshaft itself) for greatly increased valve overlap when power is needed.  The result is a significantly increased mid-range (about 15% or more) and moderately improved top-end over a similar engine with static intake cam timing. But, if you took your 20V and just left the cam 30 degrees advanced you would experience a very smelly exhaust, poor fuel economy, and decreased power below 2,200 and above 6,500 rpm or so, depending on the engine's state of tune.  By utilizing a system to dynamically change this you can have your cake and eat it too.