Tap works for me and I'm picky, but I recognize that not everyone in the world lives somewhere where the tap-water is usually better than the bottled water bought in stores.
Mineral content in tap-water can vary, even between places where tap-water is very well potable. Minerals can affect taste, according to [1] (I didn't check myself).
[1] http://blogs.discovermagazine.com/d-brief/2014/06/06/the-sec...