Why do many homes in Arizona have AC units on the roof?

Answer Arizona roof air These are probably evaporative coolers, also called swamp coolers. They cool by evaporation. In dry climates they are very effective. They cost almost nothing to operate. Cool air falls and by being on the roof the unit is exposed to the sun for the longest time. --- Az resident editing this to clarify the answer given. Yes, many homes have "swamp coolers" on the roof and for the reason given. However, many homes built in the 80s and 90s also had the AC unit/heat pump on their roofs. I have two units on my two story homes roof.