A PCV is a one-way valve. It takes the crank gases to the IM. It's a breather for crank case.
Now, those crank and head gases are not dry, they have suspended oil particles in them. That's why breather pipe from engine has hot water line going along side: to keep those gases warm, so that they may not stick to the intake pipe.
The suspended oil is not exactly healthy for your engine, it causes all kind of problems like MAP/MAF failure, lowering fuel octane to cause detonation, brunt valves, carbon deposits etc.
An oil catch can's job is to trap those oil particles, and return cleaner air to the engine.