Why do they say you shouldn't feed newborn baby water?

Newborn babies up to 6 months get all the hydration they need from breastmilk or formula. They do not need anything else. These liquids also work on quenching their thirst and giving them the nutrients they need. Water has no nutritional value and if taken instead of milk may make the baby full and thus take the place of the nutrients the baby needs and gets from milk.