When we bought our first house - it was 20 years old at the time - we had our ducts cleaned - it was a power vac truck.
The house had floor registers - so anything could fall into them.
The service was very cool - showed you what they pulled out.
There were Lego pieces in the ducts, a few coins (we kept those), some dead bugs and about 7 pounds of accumulated dirt.
Anytime windows are open - dust and pollen can come into the house, ducts circulate it and it can settle inside the ducts.
The prior owners were smokers - so that residue was in the ducts too.
The whole house smelled so much better and airflow improved very much after cleaning.