Two things come to mind here. One: There may be a buildup of undesirable compounds in the water that will not register on water tests. I imagine a good skimmer would keep most of these under control though. Two: It's a good way to replenish trace elements. Both fish and inverts need certain trace elements to stay healthy. You can dose this stuff of course and probably keep it within reasonable parameters. I do water changes on my tank and everything seems to perk up some afterwards. I know there are people who have kept tanks without changing water, but I think it's a good idea to do it. JMHO.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.