Hot Liquids, especially aromatic herbal teas and broths, work immediately to clear sinuses. When there is a fever, hot liquids encourage sweating, helping to cool the body without interfering with the fever's germ fighting work. When you have a cold, drinking plenty of fluids dilutes mucus, so it can drain. That relieves sinus pressure, and you feel better right away. 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.