Water companies are primarily interested in keepin the pH above 7 at all times, to reduce any erosion of copper piping, which is in everyone's house now. To do that they usually increase the KH of the water. And, of course, they add chlorine and ammonia (sometimes) to prevent bacteria from growing in the water. GH is normally whatever the basic water supply has - very low for river water, very high for well water, and the magnesium and calcium content depends on the type of rocks the aquifer is in. Customers for water tend to prefer low GH water, because then soap works a lot better, without the scum you get from hard water. When people add water softeners to their household water, they are trying to get rid of calcium primarily.