Every time your furnace or AC runs, air is pulled through returns, across a filter, through the blower and coil, and pushed back out through supply vents. Anything on the interior surface of that path — dust, pollen, pet dander, drywall powder from a remodel, mold spores — becomes part of the air you breathe.
The EPA has consistently ranked indoor air quality among the top environmental health risks, and homes are often 2–5 times more polluted than the air outside. Filters catch a portion of what goes through the system, but they can't catch what has already coated the interior of the ductwork.
Duct cleaning done correctly — source-removal, with negative air pressure and agitation, per NADCA guidelines — physically removes the accumulated material so it stops recirculating. That's what makes it worth doing.