If your ductwork isn’t sealed properly, you are leaking air out of the duct and into your attic. Being that this air is conditioned air, you are essentially spending money to condition your attic space. Also, as you lose air out of your ductwork, the HVAC system is pulling air out of your living space which puts your home under negative pressure. By making sure your ductwork is properly sized, and sealed, many balancing and indoor air quality issues can be improved or resolved.

Can create a negative pressure environment which will encourage attic air from infiltrating your home
Will create hot and cold spots in your home
Higher amount of dust, dander, and other VOCs and pollutants in your living space