The other point is that we measure contaminants like ammonia, nitrite and nitrate. We focus our water changes to control these since they are typically the first toxins that affect the water quality. However, other toxins may be present in small quantities. As tank water evaporates, these toxins will concentrate. Our focus is on 'the big three', but regular water changes also relieves our fish's environment of other bad things. It is a shame someone can't give people better air to breath by doing regular environment changes.