Your water kit only measures ammonia/nitrIte/nitrAte. The nitrAte helps us measure the amount of dissolved organic waste and other pollutants (something you cannot measure with hobby test kits) in the water. The plants may or should remove significant amount of nitrAte if growing well. If you don't know how much nitrAte your tank produces, then there is no way you can tell how much total pollution is in the water. Whether a tank is planted or unplanted, water changes should be done every single week (50% highly recommended). Both fish and plant benefit from the water change. There are nutrients from fresh water that plants need. You replenish the needed nutrients if you change the water. If you're depending on your water to buffer your water, then water changes are even more important because kH is lost through time.
edit: I wrote the same thing twice..