PCV=Positive Crancase Ventalation, the reason it's there is to allow the crankcase to breathe.
think of it this way: for every bit of air that moves through the intake and exhaust manifolds, an EQUEAL amount moves through the crankcase. some of this being blow by past the piston rings.
the pcv system is there to allow fresh air to vent those gasses out of the crankcase, and the pcv valve is there to ensure that the air for venting moves in 1 direction only. the pcv valve is also designed so that if there is a backfire, that the flame will not burn back into the crankcase and cause it to blow up(yes it has happened) there is no bennifit or performance gain from disconecting or modifying the pcv system. there is a slight fuel milage advantage to having one that works because it will take unburned gas and feed it back through the engine. as far as getting oil through the hose into the intake, there is a screen baffle inside the engine crankcase to help prevent this, if one has an excessive amount of oil passing through, then the rings are worn and should be replaced.