People usually use an RO/DI to make pure water (fresh) and store it.
Then they use it to replace water that evaporates from the tank on a daily basis. The evaporation leave the salt behind in the tank, so only fresh water is added.
Or they mix salt with the water and use it to replace salt water they remove from the tank. Things in the tank use chemicals from the saltwater, so every week or two you remove some of the old water (like 5% to 20%) and replace it with new saltwater. This is called a water change. In many bigger systems with lots of coral, some people do less water changes and just supplement the water in the aquarium with additional chemicals.