Homes with central furnaces and air conditioners or heat pumps use a duct system to supply heated or cooled air to the various rooms of the house and then return the air to the system for reheating or recooling. There are many points where ducts are connected to something else - to the air handler, to other pieces of ductwork, to the returns that supply air in and out of the rooms in the house, and so on. Ductwork should be properly sealed in all these places in order to prevent the loss of heated or cooled air as your HVAC system has to produce more to make up for those losses.
A 90% efficient furnace combined with an inefficient, leaky duct system equals an inefficient system that costs you money. A typical US home loses 20% of heated or cooled air through leaks, holes and poorly connected ducts, and energy contractors report seeing some homes with 50-80% leakage. See the Energy Star website for more detailed information.
The HERO Program requires HERS testing with duct sealing both before and after the work is completed in order to show the resulting improvement.
Estimate your potential to save energy and money by installing this product, please see the U.S. Department of Energy's Home Energy Saver calculator.