simple -- rain water has a different composition than ground water .
gardeners collect rain barrels full of rain water . I put out stainless steel mixing bowls to catch the rain water. Dogs love it.
As the rain moves through the atmosphere it picks up nitrogen . The pH has been changed . Typically tap water is neutral , a pH of 7.
Rainwater is mildly acidic with a pH less than 7 . Easier to absorb .
Here is a meteorologist's explanation Rain Water vs Ground Water
CrossRoads: Rain Water.......
most tap water has chlorine , and most communities have fluoride --- which is not the same as fluorine .
wash your hair with rainwater and see how soft your hair can be.