If a significant portion of your heated or cooled air is lost through leaks in the duct system, your heating and cooling system must run longer to deliver the desired degree of comfort, increasing your energy costs. A four tons air conditioner connected to a duct system that loses 25% of the air via leaks is only delivering the equivalent of three Tons (but you're still paying to run a four Ton unit). Sealing the leaks will not only reduce your heating and cooling expenses, but may enable you to down-size the unit when the time comes for replacement.
A properly sealed duct system can also improve comfort by shortening the time it takes for the space to cool down after the system has been off or the thermostat setting raised. Eliminating duct leaks can also improve indoor air quality by preventing dust, pesticides, moisture and other pollutants from been drawn into the duct system from attics or crawlspaces.