In most of the areas the water flow and/or water pressure of the main water supply is not enough to operate irrigation system on an efficient way. It means the system should be designed with too many zones and very costly or all desired parts of the garden could not be irrigated with the system.
At this case a booster set is installed. The tank is filled by the main water and it can happen even all day continuously whilst the well-deisgned system is fed by the pump installed in this tank. Capacity of the tank makes sure the water amount is enough to supply the whole desired planted area with the suitable amount of water and the emitters working on high pressure are also able to perform efficiently.
Modern low service pumps are very reliable but there are some simple rules regarding filterizing of the water need to be followed to keep them in good condition for long time.