All internal combustion engines that run off petrol are four stroke engines. This means that the piston has to go up and down a total of four times to complete a cycle. The piston first goes down with the intake valves open creating a vacuum. This draws in the cold dense air for combustion. Fuel is then injected into the cylinder. The intake valves close and then the piston rises up towards the top of the cylinder. This compression creates an immense build-up of pressure in the cylinder. The only things containing this high pressure are the cylinder itself, the piston and the piston rings that seat against the walls of the cylinder, the intake and exhaust valves are closed as well. The pressure is so high that a very small amount of the air escapes around the piston and piston rings into the crankcase.The amount of blow-by increases as the engine RPMs rise. Also, an engine with more cylinders will have more blow-by.
Inside the crankcase you have the crank which is turning in the oil pan which is full of oil. This keeps it lubricated. Positive crankcase ventilation is necessary to ensure there isn't a build up of pressure in the crankcase. This would cause the crankcase to possibly crack under the pressure and create a huge dump of oil onto the street. The positive crankcase ventilation system removes the pressure from the crankcase and reverts it back through the intake tract via crank case vents.
This pressure isn't made up of 100% air. It will also contain a very small amount of oil as well since there is so much in the oil pan at a high temperature. This air and oil mixture is then entered somewhere after the intake system, passes through the intercooler (if you car is turbo or supercharged) and then re-enters the combustion chamber (cylinder) through your intake valves to be re-burned. The oil will actually coat everything on its way back to the combustion chamber. It will develop in the intercooler, boost hoses, intake manifold and intake valves. Just on the other side of these valves is where the combustion is taking place where the temperatures are extremely high. This is what actually causes the oil to solidify on the valves.
With the oil passing through the intercooler, it can actually coat the cooling fins which will hinder the intercooler's ability to cool the air therefore lowering the efficiency.
The build up of oil on intake valves is a problem for direct injection engines. For all other engines that use port injection, the fuel is introduced before the combustion chamber which means it flows over the intake valves and enters the combustion chamber premixed with the air. The cycle of the fuel flowing over the intake valves cleans the valves from any oil that may already be on there. The oil does not have the chance to build up on the valves in port injection engines.
An oil catch can catches or prevents the oil from re-entering the intake tract. A oil catch can is placed right after the PCV and before the intercooler. This means that a more pure air mixture will go through the intercooler and intake valves. A more pure air mixture entering the intake valves means no oil build up.
An oil catch can doesn't add any power so it is often overlooked when tuning vehicles. An oil catch can will ensure you are always running the most power possible by having a cleaner intake. Oil catch cans also look great in the engine bay and contrary to beliefs, do serve an important purpose.