I think you're assuming that the air outside the car is hot. In my previous car, a passat, you could use auto but without a/c on. On a cold day, this obviously used the heat of the engine to warm the cabin to the specified temperature, regulating fan speed dependent on the difference in temperature. On a hot day, it would do its best to regulate using outside air - basically on a really hot day this resulted in the fans being on full speed (and more than likely you turning the a/c on).Its all just comfort cooling/heating. Proper aircon alters humidity
How would it cool and control temp without the compressor running? It'd just blow hot air in your face!