A house breathes just like you an I.For example every time your clothes dryer comes on it automatically creates a negative pressure inside the home.So if air goes out of the living area it has to be replaced.....now look above the ceiling fan ....you see the cover...underneath that is a hole straight to your attic....Now the stuff on your ceiling fan you call dust...call it insulation,rat fecies or anything else that is in your attic that could be pulled in your due to a negative pressure.
As a person with severe mold allergies, I live in a the South where the climate naturally breeds molds.We noticed a difference when we had our ducts cleaned.The company used a vacumn type system and covered all of our vents with plastic that was held up by the suction. We had no damage, no leaking dust/dirt back into the house and it took several hours. I think if you use a reputable company it makes the world of difference. Now we use better air filters and also have a UV light to prevent mold growth on ours system.We 'd do it again when the time comes. From your car's air filter to your dryer...anything works more efficiently when it is not filthy dirty.That's just common sense.
Air duct cleaning service providers may tell you that they need to apply chemical biocide to the inside of your ducts as a means to kill bacteria (germs) and fungi (mold) and prevent future biological growth. They may also propose the application of a "sealant" to prevent dust and dirt particles from being released into the air or to seal air leaks. You should fully understand the pros and cons of permitting application of chemical biocides or sealants. While the targeted use of chemical biocides and sealants may be appropriate under specific circumstances, research has not demonstrated their effectiveness in duct cleaning or their potential adverse health effects. No chemical biocides are currently registered by EPA for use in internally-insulated air duct systems (see Should chemical biocides be applied to the inside of air ducts?).