Originally Posted by 1stGenCRXer

Oil catch cans are to catch oil vapor and/or oil blowby from the crank case, instead of feeding it back into the intake tract.

On a N/A car, they're really not necessary. I have one on my car, but only because there was no way to hook the PCV line up to my weber carbs.

On a boosted engine, a catch can is a good idea because it's pretty easy to pressurize the crank case and cause excessive oil consumption, especially if the PCV valve isn't working properly. The PCV valve is supposed to be a one-way check valve to keep the crank case at zero pressure differential, but as I said, if it's not sealing correctly in the closed position, then the intake tract could pressurize the crank case.