Tap vs. mineral water for drinking
The mineral content in tap water differs from area to area as well as the source. The mineral content in bottled water is regulated by the companies that manufacture them, in particular the Mg and Ca content(reference).


  Death rates tend to be lower in areas with tap water containing higher
  levels of Ca and Mg. It has been shown that deficiencies in magnesium
  are capable of producing heart disturbances, including 215,000 fatal
  heart attacks in the U.S. each year, and as many as 20,000,000 fatal
  heart attacks worldwide (reference).


Different brands of bottled water will differ in their mineral content as well (reference). The place where you stay might be lacking in a particular mineral that your body requires which is why your doctor has specified the use of a particular brand of mineral water.