Having oil caked onto your intake valves can cause knocking, loss in power, loss in fuel economy. A well designed and engineered Catch Can will catch a lot of this oil.
 
Valve rings do not create a perfect seal. If they did the pistons would not be able to move up and down. Because there is a tiny gap between the cylinders walls and the pistons, a little bit of pressure is able to escape into the valve cover area and crankcase. At the same time burnt oils are also escaping into the valve cover area. This is called “blow-by”. The contents are nasty and will eventually make its way into the PCV line and Breather.
 
The PCV line will recycle these burnt solids back into your intake manifold which will increase the level of carbon solids, and reduce your octane levels. For a boosted setup with turbo and supercharger, the Breather line will recirculate some of these oils into the intake which will go into the intercooler.
 
Of course, the intercooler works best when the interior surfaces are nice and clean. When the breather line returns any fumes with oils into the intake, this will start to coat the inside of the intercooler with oil, and reduce the heat exchange efficiencies.
 
So, the simplest solution is to insert an oil catch tank can into each line, this gives these heavy oil solids a place to enter and pool.