it allows crankcase blow-by (caused by boost on worn engine turbo cars - or high boost on good condition turbo cars), the piston rings let a small amount of boost to blow by into the sump - this is normal, but as u increase boost the amount of blow by increses also. a uprated breather system will allow the gasses to escape freely and catch the oil vapour that is also carried out with them, some allow it to drain back to the sump, some blow it into a catch pot and some just let it blow back into the air intake and the engine digests the air/oil mix - not good