When we talk about water parameters and optimum levels, this is all based around the nitrogen cycle which takes place in our fish tanks. The best way to explain this cycle is by means of an image.
As you can see from the above picture of the nitrogen cycle in a fish tank with fish and plants, the fish waste and decaying plants emit Ammonia (NH3) which turns in to Nitrite (NO2) through Nitrosomonas and eventually turns in to Nitrate (NO3) through Nitrospira. The plants will use up some of this Nitrate as well as the Ammonia however generally it must be removed regularly through performing water changes.
Technically it may be possible to never do water changes however the volume of water you would need in your aquarium compared to the number of plants and low number of fish might result in a fairly boring aquarium so water changes are always advisable to ensure the Nitrogen cycle is kept stable and no spikes are experienced.