The average forced air duct system leaks 30% of the heated / cooled air out of the duct before it reaches the room it was intended to heat or cool. A very good installation only leaks 10% of your money out of the ducts typically into an unconditioned space.
While mastic can do a lot to reduce air leakage from the ducts, keeping the ducts into the conditioned space reduces the money lost.
Getting your HVAC system efficient is a great way to reduce your wasted energy each month and will make your home more comfortable on a daily basis.