Question

what is the need to get pure water from the river Ganga

Answer 1

-There will be less water scarcity
- More water will be under irrigation
-

Answer 2

Keeping aside the Mythological and passed down stories from people, there are some scientifically proven studies that state why Ganga has a pure reputation . In a study Scientist claim that the waters of Ganga have a mysterious "X" factor that give it different Qualities that make it different in these ways 
as a "disinfectant," acts on organic materials and bacteria and kills themself-purifying quality leads to oxygen levels 25 times higher than any other river in the world
Ganga have got some special properties, which other rivers do not have. One such special property is that when you store Ganga water in a close container, it doesn't putrefying.(because of lots of oxygen)
The water remains pure while millions of people bathe and waste is dumped in it