oil breather lets/vents the sealing air out of bearing housings,
as sealing air comes from the bearing housings it is full of oil and oil consumption would be very high if oil was not recovered from air.
oil seperated from air by a rotating tube, and the oil is flung out and air passes towards centre of tube and vents to atmosphere.
old Pand W JT9s used to use a litre an hour, newer GE90s use no oil.