They relieve pressure in the crankcase. Of course this is done with the pcv valve and vacuum from the intake and the air breathers are not needed.
The only time they are needed is if you are pushing boost and pressurizing the crank case.
Also, the breathers allow unmetered air into the engine, which isn't a good thing.
More air in the combustion chamber
The thing with forced induction is that some of the pressurized air will leak past the piston rings and pressurize the crankcase. This isn't really a good thing. Normally the pcv valve will use the vacuum created by the engine to relieve any pressure, but when you are pushing boost there is no more vacuum so pcv system can also push compressed air into the crankcase instead of relieving it.
Hope that made sense. Getting up at 6:00am and sitting through 2 hours of materials science makes me a little tired and worn out.