I always assumed it was an aesthetics thing.

Some places the water tastes weird like in hard water areas they taste chalky
soft water always tastes of raspberries to me so maybe its to mask the taste make it uniform .i ma talking as a londoner as we have two water sources artesian wells and reservoirs and one is hard and one is soft and it really depends upon what system you are being served by I was in a hard water area

One story goes that it was offered to clean the drinking edge of the glass in Italy then Europe