When an engine is running, it creatures pressure in the crank case. If you didnt let this out you'd be blowing oil seals and so on. Oil vapour will obviously get into the pressurized air as its all over the inside of the engine and its hot etc, so this is fed back into the engine to burnt off as thats the convenient way to get rid of the vapour, its also for emissions reasons too. Race cars run a catch tank as they dont want these vapours be burnt off as to get max performance but race regulations state you must have a catch can and not just a breather filter, so the oil is in a safe and secure tank without the possibility of leaking out say if the car rolls or is crashed etc. Hope that makes sense.