A planted tank, with lots of plants, all growing well, will not build up nitrates in the water, if it has a "normal" fish load. But, it will probably run short of nitrates, as well as phosphates and potassium and trace elements. So, that is why we dose fertilizers. If the light intensity is low enough the fertilizers may not be needed at all. One reason for doing water changes is to avoid a build up of anything in the water. If you do a 50% water change every N days, you limit the maximum build up of anything in the water to twice the amount added to the water in N days.