Because the reef crystals salt contains the proper levels of calcium, magnesium and alkalinity. I believe it's important to test water parameters, however in my specific situation im doing frequent water changes on a tank that doesn't have any acropora or other fast growing sps corals, so my corals have been doing fine without any dosing, just using the elements added to the tank with the water change every week. That being said, I will test my major elements like calcium, alk and magnesium occasionally to make sure everything is in order.