This device is installed on the water lines on your system. If the water runs over or does not drain properly the switch will disable the system preventing it from creating more water that can do damage. This will prevent ceiling damage from potential water leaks in the attic. It can prevent foundation damage from potential leaking units in the crawl space. And prevent flooring damage from potential leaking units in the house. Without this safety switch it is very common for these leaks to go unnoticed for months which cause severe and expensive damage.