In most cases we find this switch set to the “Auto” setting.  This turns the fan on automatically when the system is heating or cooling the home.  If you set it to the “On” setting the fan will run continuously when the system is heating, cooling, or doing nothing.  The fan running will mix together the air in the home through the return and redistribute it through the house vents.  The fan uses about the same amount of power as an average ceiling fan.  This is a great thing to do if you have one room that is warmer than others or a room that is always colder than others.  This could also help those who have a two story home and one system for both floors.

To find out more visit the below link.

http://www.inspectapedia.com/heat/Thermostats8.htm