Heres a pic of the top half of the duct attached to the bike, it runs from behind the header tank above the sprocket and points in the general direction of the airbox.



It clips through a couple of holes on the inside of the frame spar, the lower part attaches to the fairing and runs from the side vent in the fairing to the upper part. As suggested by NSR Lizard I would guess it does bugger all apart from make it more awkward to remove and fit the fairing, most owners probably binned them for that reason.

If you desperately need a pair I might have some spare in my ever growing pile of mc28 parts. The mc28 left hand fairing has two threaded inserts on the inside which the lower duct attaches to, mc21 fairings dont have them and I would be amazed if the Chinese ones do but you never know. I'm told the latest Chinese stuff is very close to OEM