Originally this was often a very basic setup where a filter was placed on the top of the cam case and the pressure and vapors were vented to atmosphere.
Not only was this more pleasant for the car occupants it also meant that oil mist was not released into the air or onto the road in the case of draft tube ventilation systems.
During the normal operation of an engine the excess blow-by and oil vapors from the crank case are allowed to enter the intake system.
The oil vapors can mix with the re-circulated exhaust gasses and soot which then builds up on the intake manifold and valves etc.
It will then begin to clog up the throttle body, swirl flaps, or even the intake valves on direct injected engines.
[1][verification needed] Having a buildup of sludge can cause lower performance due to the limiting effect it has on the air flow to the engine.
Fitting a catch can will reduce the amount of oil vapor reaching the intake tract and combustion chamber.