In the recent past, health authorities have become more concerned about potable water becoming contaminated if stagnant water leaks into the system. The initial concern was lawn sprinklers. A check valve is installed where the lawn sprinkler line is connected to the potable water line. If the water pressure in the potable water system got lower than that pressure in the lawn system, the fear was that the check valve could leak. The concern was heightened by the fact that this could siphon ground water contaminated with lawn chemicals.
Then someone realized that fire sprinkler systems had stagnant water held in place by check valves, and the argument for better protection spread to them as well. The answer in many states has been to require a "low-pressure-principle" backflow prevention valve. This is s sophisticated device with a series of valves that are designed to detect a pressure reduction and keep the check valves shut. The health authorities in many jurisdictions have successfully argued that even a double check valve on a sprinkler system is not enough and that the low-pressure-principle valve is needed.
Low-pressure-principle valves are costly. They also require annual inspections and maintenance by a specialist. If you can get the water and health authorities to accept a normal check valve in lieu of the low-pressure principle type, you can save the installer and owner a lot of money. When we last checked prices, the low-pressure-principle backflow preventer for a 1-family home was $600.00. This is a very significant increase in the cost of a residential sprinkler system, so reducing it can be quite an incentive.