There is another use for those. I first noticed this problem when I went to slicks in open track events and my cornering speeds went way up.
I use mine in the trunk, it is connected to the rear axle vent tube. When on the track and cornering to the right at high speed the gear oil can slosh to the left of the axle tube and can run out the vent pipe, up the rubber tube and spill out inside the unibody, run down AND DRIP ONTO YOUR REAR SLICKS!
Ask me how I know this one! Whoaaaa....HOLD ON!
Anyway, the breather is mounted high, inside the trunk and provides a place for the oil to go while in the right hander. When you straighten out, it refills the axle tube and life is good again.
You shouldn't need it on the street. (By the way, Stock Car Products makes one that should sell for substantially less than the Canton version.