An American typically finds hundreds of odd things when he arrives on German soil. So the topic of tap water will come up sooner, or later. Bluntly, Germans don't drink tap water, period.
The reasoning? It's hard to say.
When you go to a restaurant or pub, and it's in the midst of a hot summer day....you'd really like some ice cold water to refresh yourself. A German does too.....but they will order a simple bottle of water....paying roughly a Euro for eight fluid ounces of water.
Now, I will admit....there's a wide assortment of water choices. Most Germans prefer water with gas (the bubbles). Typically, Gerolsteiner will be a high choice amongst Germans. It's a mineral water, and been around for well over 120 years.
I admit...some of this preference probably comes from German history and fear of 'bad' water. This was probably true back in the 1940s. But German today is about as pure as you get.
My advice for an American in German....on a hot afternoon? As you stop at the pub.....order the largest bottle of natural water that they sell, and just anticipate drinking most of it while you sit and refresh yourself. Don't expect any ice cubes in the glass.....that's another topic for another day.
As for use of tap water for making coffee at home in Germany? Well....yeah, they use regular tap water for that. I know it doesn't make much sense, but this is Germany.
Finally, a word to the wise about getting dehydrated and stopping at a pub to have a beer, instead of a glass of water. It's not a smart idea, and your body will soak up that beer twice as fast on a hot day....meaning that you will feel enticed to sip one beer, then a second, and then a third. It's not a smart idea.