they can serve no purpose on some application except to make noise. it is possible to mess with factory air flow sensors to. once you start getting some serious boost u dont have much choice but to swap it, there is plumb back bov made, which makes a sweet noise plus plumbs air back like the factory unit.
IMHO BOVs are a waste of money unless you are running some huge turbo or did some major mods to your STi. Also if your stock one does not vent to the atmosphere and the one you put on does then you will loose a few hp. Other than for sound there is no need on an otherwise stock or even slightly modded STi.