Backflow testing is important to make sure the backflow device is operating properly. The purpose of a backflow device is to prevent cross contamination to the public potable water. For example, if you own a pool without a backflow protection device and there is a drop in the city water pressure, the pool water can be siphoned in the public potable water system, which will contaminate it. This siphoning is the reason why many homes and buildings are required to have backflow protection to prevent this contamination.