It would be a help if you named or a picture of what it is.
If it's wind deflectors on the side windows it still blows when the window is open but when it's raining you can have your window down a bit for ventilation and don't get wet.
I have the stick on ones as I did not want them in the window channel as they might interfere with the electric windows.
They are designed to provide a small amount of ventilation when its raining with the window down an inch or so. They also reduce wind noise and draft with the window down about half way. More than half way will cause just as much draft as without but with a bit less noise.
They also have the advantage of being able to leave the window down 1/2 an inch while the vehicle is parked, this is helpful to provide some ventilation in the hot weather helping keep inside temperature down, at the same time will keep out rain and because of the smoked colour hide the fact that the windows are open.