The majority of impurities in an HVAC system are dust and other very small particles. These can be blown into your home over the course of time, which can pose a major health hazard. If you don't regularly clean your ducts and vents, mold can flourish there. Mold spores blowing into a home can cause skin and eye irritation as well as respiratory problems.
A clogged HVAC system is an inefficient HVAC system. Because the build-up of dust and impurities don't allow your system to work as well as it might, you will need to spend more money to get your house to the desired temperature.
Effectively, you'll be paying money to heat up or cool down dust bunnies in your vents. Clearing them out will reduce your utility bills – not to mention actually get your home to the temperature you want.
If you clean out your ducts or vents, your HVAC system will not only run more efficiently, but it won't have to work as hard. This will help it to last longer. This is an extension of the money argument above.
However, instead of saving you money on your utility bills, it means you'll save money by getting more years of work out of your HVAC system, potentially saving thousands.
More additional reasons to keep your ducts and vents clear: