When I had a car without A/C: enough to get a tan on my right arm from gear shifting.
When I acquired a car with A/C: In the winter to get some fresh air when I have the heat on, when smoking, in the spring when it is too cold to drop the windows, but warm enough that you want to feel the fresh air. I crack it a lot more than opening it fully now, but in the summertime when you drop all the windows it is pretty cool to open it fully and really feel the wind in your hair.