Some would say there's no reason to change water if parameters and inhabitants are all good. The thing to remember about water changes, however, is that it's not only about what you're removing, but what you're replacing, too (trace elements). I suppose you could get around that by using a trace element supplement, but I don't have any experience with doing so.
I started doing weekly water changes almost to the day, but have found that I have greater water parameter stability if I go closer to 10 or 12 days between water changes, so now I do them a little less often than I originally did.
you should do them to refresh the water and nutrients to the tank. things u cant test for get used up and need to be replenished. the ocean is constantly changing the water over and thats what the fish and animals need as well.