Keeping your home’s air duct clean will help in keeping your home’s interior clean. You will no longer find dust on the furniture even after you have cleaned the rooms. If there is dirt in the ductwork, it is commonly seen that black lines form under the doors, and near the stairwells and air vents. They can be much more highlighted on carpets of light shade.
If the air duct is not cleaned, the fine dust will keep circulating and no amount of dusting will clean your indoor items. This will also increase the need for regularly cleaning your carpet.
Read more about why your home is dusty here.