Actually thet are for when it rains.
Older model vehicles had gutters built into the windshield post and roofline so you could crack open the window in the rain and it wouldn't get in the car. Todays, modern streamlined vehicles, don't have these anymore so as you know, crack open a window in the rain... you get wet.
The "Vent Visors" cover the edge of the window opening allowing you to crack (vent) a window in the rain and also act as a sun visor along the top edge of your side windows.
I hope that explains it.
Living in Phoenix that could become very useless .I had a set on my '07 Rio5 SX the first intent was for the look but far is functionality , they made a lot of noise , so I'm just skeptical about the whole "once you use them you will never be without!!" to each his own , right?
My 2 cents
I have had them on several vehicles I have owned and I love them. Great for rainy days to let a little extra air in without the rain.
I also like them for summer, you can leave the windows open just a little (maybe a 1/2 inch or so) to let the car vent and maybe not get so hot. This helps in case you get a rain shower while parked and they also at first glance give an appearance that the window is still up. I do this all the time in the summer and never had anybody mess with the car.