I believe it is because when you are breast-feeding, a thirsty baby will take more milk which then gets thinner to keep supply up to demand. I would imagine the net result is that poor mother gets dehydrated instead of baby.
It might be a hygeine thing, in case the water offered wasn't sterilised.
Boiled water was a godsend in the middle of the night instead of making up a bottle of milk with my eyes closed. After a short time mine found it wasn't worth waking up for, and slept through. A baby who is getting enough during the day doesn't need more in the night.
Re painting metal bistro garden set