A Backflow Preventer does just as its name implies, they prevent backflow. Vacuum breakers and other backflow prevention systems are designed to stop gray/dirty water from potentially siphoning back into the water supply and contaminating the fresh water. A typical backflow assembly consists of shut-off valves and test cocks and must be tested each year.
There are a number of potential cross-connections in household plumbing systems that you have to protect against backflow. When that water leaves a purified source, and flows back into its original direction, it might become polluted or contaminated, backflow preventers are critical in preventing this from happening.
In the United States, the Environmental Protection Agency (EPA) holds local water suppliers responsible for maintaining a certain amount of purity in potable water systems. A check valve is a common form of backflow prevention. In most cases, the law requires a double-detector check, an RP device or an air gap when backflow prevention is mandated.