There are a few reasons. One, sediment builds up in the water lines and inside the hydrants. So they need to be flushed to remove the sediment. Second, if the water department did and maintenance or repairs on the system, they would need to be flushed to remove and debris that got into the pipes during the repair. And third, fire hydrants need to be tested to find out what their flow capacity is for firefighting purposes. In commercial and residential areas, we have to know if there is enough water available to extinguish a fire. This also lets us know if there are any faulty or shut off hydrants so that they can be repaired or replaced.