Also, mucus that is diluted is less likely to become infected than if it is thick and sticky. Liquids soothe a scratchy throat, and keep postnasal drainage from making a sore throat worse. The body loses vital fluid and electrolytes even from a runny nose, or sweating from fever. Diarrhea and vomiting cause much greater loses. Replacing lost fluids protects against dehydration, especially urgent for babies, for older patients, for anyone who is not robust.
Dehydration is extremely serious, and requires immediate medical attention.
Prevention is better. American babies sometimes get dehydration too. It is a good idea to keep some in your medicine cabinet. They can save a child's life. Protect your own health by doing the same - and wash your hands often.