YSK because the first hot spell is when most people discover their AC unit is broken.
Several years ago I waited to turn on my AC. This is how I found out about one of the busiest times for repairmen and had to sweat for two weeks. Now I do a test run a couple of weeks before it heats up. Same goes for the furnace at the end of Autumn.
Edit to add: YSK line (Rule 2)
Same reason why if you have part time 4WD in a vehicle you should put upwards of 10 miles per month with 4WD engaged (just try to keep it to straight roads). Check your owners manual first, but every 4WD vehicle I’ve owned has recommended it.