Saline is used for two purposes. One it is sometimes used to wash procedure areas. Second it is used to keep a patient hydrated.
In terms of saline being used for hydration, because water diffuses through permeable membranes to areas of higher to lower concentrations they water would naturally do that in the body. This is super dangerous, if a lot of water enters the body without salt to keep the concentration balance in check. A person may overfill cells with water, and can cause dangerous organ swelling and degradation. That is the purpose of saline.
