An oil breather system allows this build up of pressure to escape. ... The gases are then vented off to reduce the pressure. On older cars this simply meant having a breather pipe venting to atmosphere, but on all modern cars the breather pipe actually vents back to the engine's inlet.

In extreme cases some of the old style push-on oil filler caps can actually be blown off!

Oil breather systems allow this pressure build up to be vented. Typically they will included a separator box which will catch as much of the oil suspended in the gases as possible, and then allow this to drain back into the sump.

The gases are then vented off to reduce the pressure. On older cars this simply meant having a breather pipe venting to atmosphere, but on all modern cars the breather pipe actually vents back to the engine’s inlet.

This helps reduce emissions and prevents the car from dropping an oily residue directly onto the road.

But it does mean that instead of consuming fresh clean air, the engine is actually being forced to consume an oily mixture from the crankcase – this can cause issues with inlet pipes, throttle bodies, and other parts of the inlet tract becoming gunked up with an oily residue.

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