Also what are the Sub-Air cleaners actually used for?
What it says on the tin. Filters the air that is breathed in through the PAIR system.
Speaking of breathers, a filter on the crankcase breather is a good idea, if it has been disconnected from the air box and vented straight to air. Possibly not so much an issue on a multicylinder engine, but there will be points on the four stroke cycle where air is sucked back through this breather, possibly bringing muck into the engine.