Gym Sulkinson Fork wrote:
Why are these fitted instead of just a stationary vent like you see on
caravans?

http://www.flettner.co.uk/van-roof-vents/

I don't see the point in using the wind to power a vent, when the air
was already moving.* Surely that's like fitting a generator to your
bicycle to power a motor to make you go faster?

??? Because it creates suction where there was none before?