What exactly is the purpose of a breather filter?

 

to keep the crankcase fumes from getting to the throttle body oiling up the butterfly valve and making it dirty and sticky i think. most cases ends up all over your engine and also not smog legal. most people get oil catch cans instead. this separates the oil into the can and routes the fumes back to the intake. IMO dont get a breather

 

don't remove it. the head relies on the vacuum from the intake to pull enough pressure out.

 

i have one on my car because i made my own CAI and i didn't want to drill a hole into the tubing for the small tube that goes from the valve cover to the intake.

i believe a breather filters purpose is when you’re under hard deceleration, your intake creates a vacuum and sucks oil through your valve cover into your throttle body. when i was changing my intake i noticed the butterfly valve was covered in small drops of oil for this reason. it's really no big deal since the oil gets burned up anyways, i just put one on cause it saved me the hassle of drilling and sizing the hole so everything fit.