Why drinking large amounts of water is important ?

Marketing - Sales: If slender, our body is ~62% water. We constantly intake ; output (multiple routes). If temp ; humidity comfortable, young reasonably healthy people can survive indefinitely on as little as 0.5 liter/day up to as much as ~25 liters/day, 50 fold range. Different people choose different setpoints/habits. Like most of medicine, lots of things marketed as healthy, without evidence, to promote sales.