Dirt is an abrasive like sandpaper.
Every tine you step onto your carpet you grind dirt into the carpet fibers. This cuts your carpet just as if you were using a knife. A dirty carpet will not last nearly as long as a clean carpet, and while vacuuming helps, it's simply not enough. The only reason to get your carpet cleaned is because it looks dirty. A carpet cleaner must earn this certification through study, experience, and successful completion of a formal written examination.