Insulating ductwork correctly prevents conditioned air from getting away into unconditioned areas like attics or crawlspaces

https://powerhousecc.org/should-i-close-the-door-when-using-a-dehumidifier-dehumidifier-in-closed-room/

Insulating ductwork properly prevents conditioned air from getting away into unconditioned areas like attics or crawlspaces, enhancing general system efficiency and reducing wasteful costs.