An evaporative cooler is a device that cools the air through the evaporation of water. Evaporative cooling differs from typical air conditioning systems that use  a compressor and refrigerant gas. 
The temperature of dry air can be dropped significantly through the transition of liquid water to water vapor (natural evaporation). This can cool air using less energy. It can also add moisture to dry air and promote healthy air in your home.

• In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture and reducing energy consumption.

• In climates not considered extremely dry, indirect evaporative cooling can still take advantage of the evaporative cooling process without increasing humidity.