Some of us are lucky enough that a simple declorinator is all we need to prepare the water to create a habitable environment in our aquarium. Very often, this is not the case. Many aquatic creatures do not tolerate even slightly poor water quality or swings in the water's parameters. Water in some locations actually contains nitrates. High phosphate has been linked to algae problems, and a comprehensive algae control strategy may require removing phosphates.
City water is often unstable.
Many cities modify their treatment process several times a year, dramatically changing its suitability for aquarium usage. For the perfectionist keeping more demanding aquarium inhabitants like Discus or reef organisms, more sophisticated water treatment is often a necessity. Other ions can contribute to GH but their effects are usually insignificant and the other ions are difficult to measure. In the U.S., hardness is usually measured in ppm of CaCO3. A German dH is 17.8 ppm CaCO3.