Hard water treatment systems work by passing water through an activated carbon block that removes harmful chemicals.  Activated charcoal works well at removing chlorine, which makes drinking water safe.  However, some people have trouble getting their water to taste good without adding extra flavoring agents.  These additives add color and flavor, and chemical compounds that are bad for you. Drinking water treated with a Brita water filter tastes great because the water is free of these artificial flavors, colors, and preservatives. Further explains in this video: