The most common reason leaks occur is that the rubber parts in the valve box have worn out. Water seeps freely through the system when the diaphragm seal breaks or debris is obstructing the valve from closing. This means unInecessary water is flowing 24/7 and could be causing other issues in your landscaping.
Valves not only control the water flow to the sprinkler heads in each zone, but they also have a life span and parts that will eventually need to be replaced or repaired. Solenoids and Diaphragms both wear out and eventually need to be replaced.