You have a breather which allows the crankcase (inside the block) to be vented. Otherwise, when you have blow-by gases (the air/fuel that goes beside the piston rings) the block would build up pressure and eventually crack.

This breathing sometimes allows oil to escape aswell as air, mostly at high RPM and high LOAD times.

If this oil kept escaping you would spend a fortune on oil, so you catch it and most of the time allow the oil and air to seperate and the oil to drain back to the block, the air to be vented.