When you turn on a car ac the ac compressor engages which then puts an additional load on the engine.
That additional load on the engine requires more fuel to keep the compressor pumping so that refrigerant flows through the system and keeps it blowing cold air.
If it's not really hot I just roll down the windows because I sometimes like a nice breeze especially driving through town but on the highway I will usually turn the car ac on especially if it gets above 95 F out.
I do notice at least a 1 mile per gallon drop in my fuel mileage but when it's really hot out it's worth the extra fuel use and cost to stay cool and not sweat.
Car ac and home ac is a great invention and I will use it when needed.