Lenses are the longest-lived equipment most photographers own. A lens bought today may still be in use, and still be resold, twenty years from now, which makes its failure modes far more consequential than a body's.
Autofocus motors, aperture mechanisms and zoom barrels are the parts that fail, and they fail on schedules that only become visible with years of ownership data. This category records those mechanisms explicitly, because motor and barrel design predict long-term failure better than optical performance ever will.