It's obviously healthier to drink water if one's able to, but in cases when someone cannot drink (people with an obstruction in their throat or elsewhere in their digestive track, or some other digestive tract problem, or in burn victims who simply cannot drink enough water to keep hydrated, or they are too sick to drink safely without getting fluid in their lungs and getting pneumonia or something) it is possible to use IV's to replace drinking and even eating for quite some time.
There are problems of course with this. It is very expensive to continually monitor osmolarity and monitor rate of infusion. There's excretions of liquid into the lungs and edema (build up of fluid and swelling) in other areas of the body. There are reactions to long-term IV needle use like redness and skin irritation and infections. Nausea and dry mouth are problems as well. If you can use your digestive tract, a lack of water in it while using it will cause digestive problems, like absorptive issues and constipation.
Because of the problems, this is only used in temporary situations, or when a person has an advanced disease and is dying.