they can also add some power/torque by better tuning the intake resonance http://www.solomiata.com/airflowmtr.html scroll down towards the bottom
1990-94 Protege / 1991-96 Escort GT resonator
I have a 1996 Ford Contour 2.0L 4 cyl Zetec and it has a VERY large resonator after the MAF on the intake. This engine is noisy enough WITH the resonator. I'd hate to hear it without it.