It's not just to keep cool in summer. If you are in a part of the world plagued by mosquitos (the Mediterranean, the tropics, Southern USA), air conditioning zaps the blighters.
Mosquitos are cold-blooded and can't cope with low temperatures. Their ideal temperature is 26°c or 80°f. They have a defence mechanism, where they go into hibernation at low temperatures: 10°c or 50°f usually triggers hibernation - but they find it difficult to go from hot air outside to cold air indoors in a few seconds - their inner defences don't react that fast. Most mosquitos will sensibly fly away from chilled areas.
If you set the air conditioning at 15°c or 60°f, and let it run for two hours to chill your house sufficiently, any mosquitos that have got inside will drop dead.
Air conditioning is one of the miracles of the modern world. Prior to it's invention, people used to think that people in the Southern USA, Med and tropics were just plain lazy because their output was so sluggish.
But they were not just coping with debilitating heat and humidity, but battling diseases borne by mosquitos and ticks.
Air conditioning changed all that, protecting people in a chilled interior that the blood-suckers struggled in.
I was inspired to write this piece by the news that Spain has banned people from setting their air condiioning below 27°c or 81°f to save energy. In other words, making interiors perfect for mosquitos. A generation of Spanish bureaucrats lack the knowledge of their ancestors. I guess malaria will make a comeback.