Infants with severe diarrhea can die of dehydration, and can reach a point where plain water doesn't help. It's one of the most common causes of death in the developing world. Water with precisely the same ration of salts and sugars as your body fluids can re-hydrate infants who would otherwise die. It's listed by WHO as an "Essential Medicine" and probably saves more lives then chemotherapy or lipitor. Of course, you can take water, boil it, and add sugar and salt to it...but you have to get the proportions just right.
https://en.wikipedia.org/wiki/Oral_rehydration_therapy
