Yeah mkivsoopra more or less hit it on the nose there. They act to deaden the intake sound as well as store air for increased throttle response. You would usually say "straight through" Airflow is better BUT our cars have that damn MAF thingee. This means that when we need more air it has to travel through the MAF first so in order to get better throttle responce those lil boxes were added to store air so that it didnt have to go through the MAF again right away.
I took them off and i really dont notice a difference but it does sound better. However as far as performance i really dont notice a difference in power its just that they supposedly held air for better throttle response. But i think its worth the difference in looks and sound. After all only a matter of time before there is a turbo there anyhow