Humidifiers are normally used in the winter months when the furnace or heater is in use. When you are heating your house, you are essentially removing all water from the air and drying it out inside. You need humidity in the air to prevent dry skin, and to prevent colds. Dry air irritates the nose and causes much more infection to set in.
A humidifier is used to add moisture to the air. In the winter, air can be dry. This can cause nose bleeds, breathing difficulties, and static electricity in the house. Running a humidifier helps put moisture back into the air.