Answer: Because they get dirty!
In addition to normal accumulations of dust and dirt found in all homes with air ducts, there are several other factors that can increase the need for regular HVAC system cleaning:
Some occupants are more sensitive to these contaminants than others. Allergy and asthma sufferers, as well as young children and the elderly tend to be more susceptible to the types of poor indoor air quality that air duct cleaning can help address.
All ducts are not created equal therefore we carry different machines such as negative air machines with compressed air & rotary with brush systems.