Coffee is 98% water.
If you want to brew coffee with perfect water, this is the solution. Add one stick to a gallon of distilled water, shake the jug, and voila! Perfect water for coffee!
Aside from bad tasting water making coffee taste bad, even good tasting water can affect the extraction of coffee. Too high of Calcium can mute acidity, to hard of water can underextract causing sour flavors, distilled water would over extract coffee leading to bitter flavors. It really has a huge impact.