Simply said, keeping the cooled or heated air inside the duct at the temperature developed inside your unit(s) until it reaches the diffusers in your home, your systems are operating more efficiently saving money. Your not heating or cooling you attic or crawl space.
Save some money.
If your duct is not internally insulated the outside material will become cold. Imagine a cold soda can. When it is hot in the space the can occupies, the can will sweat. That's condensation. The same principle applies to duct. If the air inside is cold, the duct will sweat. Internal insulation keeps the outside of the duct from getting cold. External insulation has a built-in moisture barrier. If installed properly this barrier will keep the moisture from reaching the duct and developing issues such as rusted out duct work or MOLD. Not to mention water damage caused by dripping condensation.
Protect your investment