If my understanding is correct, in a standard combustion engine as you press the gas pedal the speed of everything increases: the cam shafts spin faster (and thus the valves move faster), the pistons move faster...EVERYTHING goes faster. That sounds like an optimal system in theory, so what is the benefit of having the valves be able to be intelligently out of phase (for lack of a better phrase) with the rest of the system?
Thinking of time, the valves are open longer during engine idle and much less at higher engine speeds. Now think of a small block chevy where we can change engine timing a little bit advanced or retard which affects power and performance. To make a bigger change, another grind of camshaft has to be installed. This grinding of camshaft is called profile - the characteristics of the lobes size, ramps and direction which work at an entirely different fashion. With variable valve timing, the computer can advance or retard a cam to adapt to many different profiles to provide the best power, torque, economy and lowest emissions possible for any given condition such as cruising down a highway, climbing a long tall hill, or driving light to light through the city. Almost an infinite amount of profiles without swapping out a single camshaft. Seems alot easier and sure is alot cheaper, and we don't have to do the work anymore. Systems are pretty reliable as they typically use engine oil pressure that is directed by a solenoid run the by the ECM on a duty cycle which allows oil pressure to the advance side of a cylinder on the end of a camshaft where the cam is rotated ahead of its resting position thus changing the time the valves are open and when they open. The cam is spring returned to resting position ( some may be hydraulic?) Hope I explained this so it is clearer than mud.
Modifying camshaft timing under changing engine demand provides better balance between the following performance concerns: Engine power output Fuel economy Lower tailpipe emissions
Valve timing is a compromise. You can optimize valve timing for a certain RPM, but it will be wrong for every other rpm. Advancing a cam helps bottom end, and retarding a cam helps top end. Actually changing duration and lift is more difficult, but can be done, and IS being done. The benefit is to widen the torque curve, helping both power and economy
Because o a typical engine without VVT, the valve opening is optimized for maximum airflow near peak power, whatever specific RPM is chosen to optimize for, so if it is a high output engine, it may not have good low end torque, or if it is optimized for economy, it has good low end torque but poor top end power. What VVT does is allow the timing and possibly the lift as well to be varied with RPMs, so perhaps at any given RPM, the valvetrain is dynamically altered to give the best airflow for that RPM. This allows for a car that can get really good MPG if you drive it gently, but will also perform really well if you stick your foot in it. In theory, if you can vary the valvetrain through a sufficiently wide range of lift and duration, it essentially replaces the function of a throttle plate. Reducing valve lift and duration is like closing the throttle, and increasing it is like opening the throttle more.
