The action of the pistons going up and down alter the pressure under them in the crankcase, so the crankcase needs to vent otherwise it'd blow the casing apart.
This vent is connected to the air intake so the oily vapours get sucked into the engine to be burnt rather than polluting the atmosphere and dribbling oil over the road.
Great theory, but in practice it can be a bit of a headache for the intake side of the engine.
Diesels, due to the high compression ratios create a lot more pressure under the pistons, so tend to need to vent a lot more than a petrol engine, so tend to have larger breather pipes.
Cold starts (where the pistons and rings haven't yet expanded) and a bit of piston/ring and bore wear (so a bit extra piston blowby) and they tend to start throwing a bit more oily vapour out.
Trouble starts in the intake side as the oil coats up the pipework, intercooler and inlet manifold, at this point the EGR valve operation allows exhaust gases back around into the inlet side which is full of soot, so the oil and soot mix to create the worse gunk know to man which then clogs up the inlet and jams up the EGR valve.
Big commercial diesel engines tend to run with oil separators that swill out the oil and return it to the crankcase.
Certain VW TDi's are notorious for breathing heavy and owners often fit catch tanks, but there are not the usual Ebay specials, but large ported, baffled tanks design for diesel engines.
I ran one of these for 120k on a 2.0 TDCi.
Worked perfectly.
http://www.allardaluminiumproducts....lf-seat-skoda-audi-tdi-vag-group-engines.html
