OK, so now there is yet another issue I need to know about my car. What is oil blow by, why is it a unique issue with a SC, and how do these catch cans solve it?
Oil blow-by is the little bit of "stuff" that gets past the rings in the combustion chamber - the stuff that burns and is supposed to go out the exhaust when the cylinder is done with it. All cars have a bit of blow-by, but as things age and wear (the rings on the pistons), you get more blow-by ("leakage").
This blow-by is a mixture of oil, fuel, and combustion byproducts - it appears as a "vapor" that contains the oil, fuel, and the gasses from the combustion.
It is normally sucked into the intake (and then into the cylinders) where it is burned off without any problems.
But with a supercharger, the intake passes through supercharger (where it gets hot) and then inter-cooler (to cool it back down) before it gets into the cylinders. The problem is that the blow-by can leave an "oily film" on the inside passages of the inter-cooler, which acts as an insulator and reduces the inter-cooler's ability to cool the intake.
A catch can helps prevent this by passing the blow-by into a big open can where it gets a chance for the oil/fuel to settle out of the vapors and collect in the bottom of the catch can. The gases continue to pass through to the intake (or some people vent it to the air) where they continue to get burned - but the oil doesn't leave a film on the intake passages or the inter-cooler.
But you have to remember to occasionally drain the oil/fuel collected in the catch can as it accumulates.
A catch can is very helpful for a supercharged/inter-cooled car. But really isn't necessary for a normal (non-supercharged) street driven car. If you drive your non-supercharged car on the track a lot, then you may need one as a car that is at high revs much of the time will be generating more blow-by than a normally driven car. Then again, frequent oil changes will remove the blow-by "gunk" that collects in the oil (hence the reason for changing your oil before or after a track day).