A humidity controller just turns a device on or off when the humidity hits a certain point, just like a thermostat does for temperature. These can be used to control dehumidifiers or humidifiers -- of course, nowadays, most dehumidifiers have a controller build in. In most cases, you don't need one -- people generally have a problem with high humidity, so all you need is a somewhat modern dehumidifier. However, if you have problems with low humidity, they can be useful to control a humidifier, since these usually -don't- have controllers built in -- and from what you say, it looks like you might want one.