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you would recirculate the air back in so that you keep the same amount of air in the intake as the car thinks it has.
explanation:
The MAF measures how much air is going into the engine. When the blowoff valve releases pressure, that removes some of the air that the MAF has already measured and sent to the ECU. The ECU thinks you have X amount of air going in, so it adds X amount of Fuel to accomodate the volume of air. If you loose some of the air and the ecu doesn't know about it, then the same amount of fuel will still go in the cylinder. This cause the car to run rich when you let of the gas. Which makes for a crappy idle.
If you recirculate, then you keep the same amount of air in the system, and the MAF doesn't do any lieing to the ECU.
Its not necessary to do this, but it will make the car run a little better at idle, and will keep it from dying when you let off the gas.
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