On most engines hot gasses from the crank case are recirculated to the inlet. While this is probably fine for moderately daily drivers, on hard driven or high boost cars that experience more blow-by, build-up of by-products from combustion can start to become a problem with carbon and oil sludge building up in places you don t want it. It can also in theory reduce power by raising inlet temperatures. A popular solution to this problem is to pass these hot gases through a catch can. This separates air (good) and oil/carbon (bad). The air goes back to the intake system. The sludge stays in the can. Everyone is happy! For really extreme applications Mishimoto even offer cans with baffles in to stop the collected sludge slopping about during extreme cornering manoeuvres.