The wind makes them spin and they function as a centrifugal air pump, taking air out of the house.
They are pretty good at keeping the rain and birds out.
me too? 
I think they are called ‘ventilators’.
Crap, original message didn’t come through. But you’ve gotten the heart of the question.
Why would people want essentially uncontrolled venting of air in their houses? Why would they want it only when windy enough to spin it? Or is this mostly for venting attics where these wouldn’t matter so much?
In Australia where temperatures in summer are pretty darn hot, they are used to vent the build up of hot air from the roof space.
Basically, when the roof space fills up with hot air in the summer, it makes the aircon work harder and if you don’t have aircon, really keeps the house too hot at night.
The vent enables the hot air to escape (either through convection or suction when the wind blows) and generally keeps the house cooler.
I’ve seen them called “wind turbines.” I call them “chimney monkeys,” a term from a children’s book my aunt gave me many moons ago. (Chimney-Top Lane, by Gunnel Linde, as if you care.)
And here I thought they were used to force cool air (fromthe breeze) into the house…
(Owner of two vents 
)
Bit of a hijack:
They have loads of these things around a shopping park in NE England. There are loads of the buggers scattered about the grass around the carparks and even infect the surrounding land. Why would they be here?
The carparks are just outdoor areas, incidentally - there are no underground (or above ground) storeys.
Those things will work with or without wind… it’s just an attic chimney with little spinning blade deal on top. When it’s really hot the air rising will turn the ball. When there’s a bit of a breeze, it’ll also turn and draw some air up and out (whether or not it’s hot air… everyone I know just leaves them in place during the winter so they suck out all the warm air; not the best idea IMO).
A friend of mine has a couple on her roof that no longer spin, and was told that she needed to get them replaced (to the tune of ~ $160). We mentioned that was BS; they are open at the top and hot air will still rise out through the vanes whether or not the ball turns. A non-turning one just won’t draw extra air out when the wind blows.
The spinning in the wind also keeps rain from entering the attic area through the louvers on the ball. You generally don’t want rain in your attic and on your insulation.
Back in Oklahoma, where summers are hot and winters are cold, most houses had these to get the warm air out. I also remember as a teenager, my dad made me climb up on the roof as the weather cooled to tie plastic bags over them so that the warm air wouldn’t escape.