Originally Posted by Buck Spunni

Goopy oil vapour and condensate are routed out of your crankcase into the intake so they can be mixed with incoming air and burned off to meet emissions standards. The catch can intercepts this grunge and prevents it from gumming up your throttle body etc. Instead of burning the goop in your engine you empty the can periodically.
Do you really need one? No, but it certainly helps keep the engine running clean.