Backflow Preventer is very important. Backflow is a reverse flow of water from your house pipe to the main water supply pipe. As opposed to the conventional flow of water from the main supply pipe to your house pipe. This house pipe could be your drinking water pipe, irrigation pipe, tank pipe, etc. Why does the water flow backward? It flows backward due to Siphonage when the pressure becomes low in the main water supply pipe. As well when the water of the house pipe, connected to a water tank immersed in a water tank flows back and mixes with the water of the main supply pipe.