What happens when the air passes through the intake (or exhaust) is complex. But one major factor is that pressure waves resonate up and down the intake. These are created when the valves open and close. Timed right, these pressure waves can assist the air entering the intake. And the length of the intake trumpets is important, since it determines how far the pressure waves have to travel. Since the idea is to get as much air in during each intake stroke, this makes a significant contribution. But engine RPM's rise and fall, and such do the pressure waves.
Variable length intakes are designed to accomodate chages in RPM, you get more power, a broader torque band and a better running engine if you can tune the length of the intake based on RPM.
http://www.eng.ed.ac.uk/~jchick/Y5/thes ... ml/pt7.htm