Dirt is abrasive - like sandpaper.
Every time you step on the carpet, you grind dirt into your carpet fibers. This cuts your carpet, just as if you had used a knife, causing it to wear out faster. A dirty carpet will not last nearly as long as a clean carpet. While vacuuming helps - by itself, it's simply not enough. When you and your family members come into your home, you carry those pollens, bacteria, and chemicals in your hair and on your skin, clothing, and shoes.