Sometimes called an oil-air separator, these items are a useful addition to any car, but especially CA18DET's as they are known to have quite a bit of pressure in the crank case. The catch can acts as a reservoir for the oil blown out of the outlets of the tappet cover. Instead of being tapped back into the intercooler piping before the turbo, and also directly into the plenum as is standard procedure, the 2 lines both run into the catch can.
Any oil that comes out of these lines then is trapped in the can, with no oil being recirculated back where it isn't needed. Every few months its a simple matter of letting the excess oil drain from the drain plug at the bottom of the can. Alternately you can run a line back down to the sump to let the oil drain down there automatically.
This avoids the common problem of oil all through the intercooler piping, intercooler plenum and vacuum lines, and ensures that only clean air is circulated in these areas and eventually burnt in the motor.