Zones are used in sprinklers because of water flow rates provided by the water source (Gallons Per Minute or GPM) and because of the layout of the property. Certain areas may require more control by the property owner so a separate zone will be put in that area.
An average city water house supplies roughly 14 GPM. That is why most residential sprinkler zones have 4 rotors (3 GPM per head x 4 = 12 GPM) and a mist zone usually will be no greater than 12 mist heads (1 GPM x 12 = 12 GPM).
Assume you have a very sunny yard, but the side of the house is very shady. You would not want the shady area to run with the sunny areas because the two have different watering requirements. If the areas are zoned differently, then the owner can water each area as necessary.