Because your stock one is fine up to around 20psi. It recirculates which makes your car happy. And thusly makes you faster since you don't run pig rich during shifts due to missing air from teh recirculation which is factored in after the MAF in the ECU. When you shift, your ECU will fuel your car for the recirculated air that it is expecting from the recirculating BPV. IMO, don't waste your money.