They keep the air you breathe wet. Your sinuses can't drain right if they are infected and swollen. Your doctor can tell if you have a sinus infection with a physical exam, a laboratory study of a sample of your nasal discharge and X-rays of the sinuses. These work to clear the infection and reduce congestion. A cool-mist humidifier can help.
Wet air helps make mucus thin.
You can put a warm washcloth or compress on your face, too. This can help with the pain. Take aspirin or acetaminophen, ibuprofen or naproxen sodium for pain. Use nose drops only for the number of days prescribed. Repeated use of them creates a dependency.