It is actually there for oil vapors coming off the cams and valve train.
Just sticking a breather filter on the valve cover will do nothing good.
(Granted, just having a breather filter will not do any harm either.)
The system work by using the sucking of the intake to help draw the vapors out.
My breather filter was on only temporarily, but now I have a air/oil seperator that connects between the valve cover and intake.
