Backflow devices protect the drinking water supply from contaminants. For example, on an irrigation system. Sprinkler heads sit in the yard exposed to fertilizers, animal feces and other hazards. Or fire suppression sprinklers. Water sits in the pipes ready to pour down on a fire. For years that fire never comes. The water in those pipes gets pretty gross. Backflow devices make sure those contaminants cannot enter yours and your communities drinking water. 

Backflow devices are manufactured in various configurations but a few key items are relatively the same across them all. All devices have check valves and some have relief valves also. Those are made with springs and rubber seals that create drip tight seals. Ever year those parts are tested to confirm they are in fact preventing the flow of contaminated water and they are working to the required standards. Eventually those springs get weak or the rubbers wear down. Occasionally debris get lodged in the assembly. Testing diagnoses any failures in a backflow device. In these events cleaning or repairs are required. 

*There are additional variables inspected and recorded to complete a backflow assembly test. For complete testing requirements see the USC Manual of Cross-Connection Control 10th Edition Manual.