Sprinkler backflow preventers are used to protect water supplies from contamination caused by pollutants on your lawn and fields.  Things like pesticides and fertilizers, if mixed with the potable water supply, will contaminate that water and make it unsafe for human consumption.  That is why a sprinkler backflow preventer must be used, must be tested annually and must be maintained annually.

Backflow occurs when there is a sudden decrease in the water pressure inside your pipes.  If not checked immediately, water will reverse direction and flow backwards into the main water supply.  Thus the need for a sprinkler backflow preventer, to keep this backflow from happening until the water pressure can again be raised to proper levels.