usually people use them because the tube has no where to go back into the home-made intake. All it's doing is exhausting unburnt vapors back into the intake (when it's connected)... so the advantage of putting a filter on the end of an exposed tube is because it doesn't smell so great (it'll vent into your car), and so nothing gets into the head.
oh, and you can buy the filters at Pep Boys.
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