Short answer
Steam cleaning removes the dirt, dust mites, dander and trapped pollutants that vacuuming leaves behind, and it extends the life of the carpet by getting the abrasive grit out of the pile.
What does carpet actually trap?
Dirty carpet holds pollutants from many sources: pet dander, dust mites, allergens, and particles tracked in from outside and from work. Professional steam cleaning can also lift trapped odors such as cigarette smoke. Vacuuming removes the loose layer, but the finer material works down to the base of the pile.
How does steam cleaning make carpet last longer?
Grit is abrasive. As foot traffic pushes it deeper, it grinds against the fibers and shortens the life of the carpet, and the deeper it goes the harder and more expensive the carpet is to clean. Cleaning before the carpet is heavily soiled keeps that damage down.
How often should it be done?
To keep a carpet looking its best, deep clean every 12 to 18 months. Adjust for your household: pets, children, how often you vacuum and the color of the carpet all change the right interval.