Air flow for HVAC can become restricted and this causes your HVAC system to run longer to get to the desired temperature.
This causes your HVAC to work harder which results in a higher electric bill. Your HVAC system can also break down sooner which can also get expensive.
Dirty vents and air ducts not only allows dust back into your home but also dust mites, pet dander, mold and other allergens.
Inside your HVAC system, fine dust collects on air duct surfaces, vents, motors and coils. Flakes of dead skin, hair and pet dander catch onto the dust, and before too long this process creates a dense, mat-like environment where the bacteria, mold and dust mites can flourish.
If your vent covers look dirty this means that the inside is dirty and needs attention. Especially if it gets humid which will harden everything and make it difficult to clean. With clean air ducts you will definitely notice the difference.
You would be surprised if you actually traveled through the air ducts in your home. They are everywhere and are used by your heater and air conditioning system. Some places have dryer vents and exhaust vents, We find everything in these ducts from animals to shoes. Some even totally blocked by dirt or other substance.