What /u/MaybePenisTomorrow/ said, plus your shower water is not hot enough to vaporize or beak down the minerals and additives like chlorine in tap water.

You can mix in distilled water into your VG, but avoid tap water because of the impurity.

Probably cause of the Calcium in tap water, among other treating chemicals that can be harmful when inhaled (fluorine maybe). But I'm not expert.

Think about the inside of an electric kettle after it's been run a few times. If you have hard water in your area, then that limescale could potentially build up in your atomizer. God knows what that stuff would be like to vape.

I imagine because it has a distinct flavor. I can make a recipe that includes tap water but if you try to replicate it, you will end up with something that tastes completely different. Why? Because we are in different towns, our tap water is going to have a very different chemical makeup because of different lakes, springs, water sources, treatment facilities with different methods and chemicals used to make the water safe. That is why people use distilled water, because distillation purifies the water like nothing else can, it brings it to it's natural PH and gets rid of virtually all contamination.