It's much easier to prevent water quality problems by doing regular water changes than to deal with parameters that are out of whack.
Not only does doing a water change take out bad stuff, but your corals and other animals are constantly consuming minerals and elements from the water column (ever see test kits for iodine, iodide, molybdenum, magnesium, etc and wonder "what the heck are those for?"). Synthetic salts that we use to make our water contain a balance of those minerals that are required for coral health and growth. Without water changes, those elements get depleted, but doing a water change replenishes them.