Secondly, if the front-mounted air intakes feed into a sealed airbox you can get a small forced induction effect as the bike goes faster-not unlike a small turbocharger. The incoming air pressurizes the airbox, which crams more oxygen into the engine at each piston stroke, again making more power.
This "ram-air" effect was first used by Kawasaki on the ZX-11 in 1990, and has since been used on almost every sportbike produced. The actual power gains are fairly minimal, and only show up at very high speeds. But hell...they're free. Besides, big frontal air intakes look super-cool.
Lastly, an airbox can improve power delivery by managing the pressure waves in the intake system. Similar to an exhaust pipe, the engine makes a series of high and low-pressure sound waves as it opens the intake valves into the engine. These waves stream outward from the carb or throttle body intake and will bounce off whatever they find. If the intake throats are simply opened to the surrounding air, the waves will be dissipated and not really do anything (except make a lot of noise.) But once you add an airbox to the inlet system, you can use the waves to do something more useful. Carefully-sized chambers (called Helmholtz chambers) in the airbox will resonate at certain frequencies, and can actually help improve the airflow into the inlet at certain engine speeds. Who knew?
Race Technology
Ducati's MotoGP team has recently shown how important airboxes can be, by building its entire Desmosedici GP9 racebike around a carbon fiber airbox. The super-stiff composite construction has totally replaced the steel trellis frame used on previous versions of the racebike, and the airbox is part of the chassis.
This is clearly an extreme move, but it makes sense. It allows the maximum volume for the airbox, saves on weight and complexity and makes for a mega-stiff frame. It's not the first time it's appeared on a bike either; Kawasaki's ZX-12R Ninja had a similar design with the air filters and airbox mounted inside the aluminum monocoque backbone frame.