Soil, dirt or sand when viewed under a microscope reveals sharp jagged edges. Many of us have had the unpleasant experience of getting sand or dirt in the eye, the abrasion is a severe irritant until flushed out. Even a small pebble in the shoe can be too sharp to bear for very long. Though these are rather unpleasant images, they do well to illustrate that in the same way, dirt or sand left in carpet even in small quantities is an abrasion that constantly 'cuts' or scratches carpet fibers especially in high traffic areas. The carpet wears down faster, resulting in a worn, damaged appearance.
Replacement becomes necessary sooner.
Clean carpets make healthier environments. These result in unpleasant odors and are laden with harmful bacteria that in turn can lead to more serious hazards and severe damage to the carpet, under padding and even the floor. Many of the same or similar but unseen and undetected contaminants can be tracked in by pets or under the shoe, unnoticed spills from the kitchen floor, tiny insects, etc. In brief, the sources seem limitless. Carpets properly cleaned can alleviate these concerns and perhaps in an indirect way even reduce medical costs.