Sacramento summers are dry, dusty, and long, and every open window and every trip in from the yard brings fine Valley dust inside. Carpet acts like a filter: it holds that grit at the base of the fiber where a household vacuum can't reach, and by September there's a season's worth of it in every traffic lane.
Add smoke. Wildfire season leaves a film of fine particulate on every soft surface in the house, and carpet and upholstery hold it longest. That's the faint smell that lingers after the air clears, and it doesn't vacuum out.
Hot-water extraction is the fix. A pre-spray breaks down the oils that bind the dust to the fiber, agitation works it loose, and high-pressure rinse and extraction pull the soil and the water back out. Done properly, there's no sticky residue to attract new dirt, and the carpet is dry within a few hours.
Fall is the right time because the windows are closing and the holidays are coming. Extracting a summer's worth of dust before the house seals up for winter means cleaner air indoors and carpet that's actually clean for guests, not just vacuumed. Pair it with sofa and upholstery cleaning and the whole living room resets at once.