Water changes do more than remove nitrate (there shouldn't be any ammonia or nitrite). They remove all the other things we cannot test for; all the other things that are excreted by fish and things like hormones secreted by the fish. Unless they are removed they will build up in the water. I once worked in a hospital lab and we tested urine for all sorts of things. Fish excrete similar, if not the same, waste products - and there were many more things in urine that we didn't test for.
As for topping off due to evaporation, if this is done, a form of pure water such as RO water should be used. Tap water has minerals dissolved in it and these do not evaporate. When topping up is done with tap water more minerals are added. Over time the tank water gets harder and harder if tap water is used. With RO, the pure water which has evaporated is replaced with more pure water.