An engine (with properly made up joints) would probably not leak oil even if it were simply allowed to vent to atmosphere. But that isn't the primary purpose of a PCV system, and it's not even the secondary purpose.
Even the tightest engines will have some degree of blow-by (i.e. combustion gas leakage past the piston ring/cylinder wall interface). The blow-by gases need to be continuously scavenged from the atmosphere within the crankcase. If these gases - which contain acid and water - are still in the crankcase when the engine is shut off, they will cool down and condense into liquid. This allows them to trickle down and mix with the lube oil.
Given a sufficient number of heat up/cool down cycles, they begin to form a significant percentage of the engine oil volume. The oil's lubricating properties begin to drop off, the acids in the oil begin to etch into the metal components of the engine and the water forms an emulsion that clings to the surfaces. An emulsion is a mixture of water and oil - it's that white slimy stuff you see inside valve covers.
If this process is allowed to continue, it gets worse. The acid will eventually cause pitting of the surfaces of the steel journals in the bearings. And the emulsified oil gets baked into sludge. The sludge accumulates in the oil passages and begins to restrict flow. The reduced flow, along with the pitting caused by the acid, starts to effect the ability of the bearings to maintain an oil film between the babbit and the rotating steel journal. The babbit begins wear away and the tight original clearances are lost.
Given enough time, the engine will be choked with sludge and bearing clearances will become excessive. This will happen even if you are diligent about changing the engine oil.
The way prevent this all from happening is to continuously remove the blow-by gases. And the most effective method to remove them is by scavenging the crankcase atmosphere with a constant flow of fresh air.
Rather than using an additional device to force the fresh air into the crankcase, the PCV system uses the vacuum created within the intake manifold to draw the blow-by out and into the combustion air stream. This is done via the PCV and it's associated hoses.
An atmospheric vent is necessary in a PCV system as well. It provides a path to allow an unrestricted flow of incoming fresh air. That air replaces the scavenged crankcase gases and prevents a negative pressure from developing within the crankcase. Scavenging is most effective when done in a diagonal flow. That is why you'll almost always see the valve cover vent on the opposite side and end of the PCV.
The two main benefits from this system are:
1. Harmful blow-by gases are removed from the engine's crankcase and combined with the incoming combustion air. Ultimately they are burned within the combustion chambers.
2. Since the rate that the gases removed by the manifold vacuum are replaced by a flow of fresh air drawn in at the same rate, a natural balance is always maintained. Crankcase pressure is always equal to atmosphere and in theory nothing goes in and nothing goes out.
A third benefit is a slight reduction in emissions - those blow-by gases that would have been released to atmosphere, but were instead burned in the engine.
