Air is introduced through the inlet tract, forced induction or not, mixed withfuel by fuel injectors or carbs and pulled into the cylinder by the vacuum of the intake stroke. It then gets compressed by the upward piston stroke and the mixture is fired by the ignition system. This forces the piston downwards and the stroke produces the power to turn the crankshaft. The rotation of the crank then brings the piston up and the final stroke upward stroke exhausts the burnt mixture, dependent on the valve opening.
The exhaust gas can also help scavenge an incoming charge by creating a better vacuum in the cylinder and hence the advantage of longer overlap cams which promote greater vacuum amongst other things.
In short air in equals power out.
This is the reason that we have forced induction in the first place. The more fuel/air mix we can flow into the cylinder the more potential for bigger and better combustion, or power output. How is power produced by a car engine in order to move the car. Air needs to be introduced to the air filter by the cleanest means possible and the air needs to be as cool as possible. On a series production car the manufacturer often has a problem in where to place this unit within the confines of the underbonnet are whilst maintaining adequate air flow to the unit.
To increase performance on street cars we need to modify the air filter/air box area to introduce more cool air to the engine without it becoming so large that the installation is a problem.