Dry soil from a rug
Rugs should be cleaned because they collect soil, sediment and pollutants just like carpets do. All this grit and sediment is very damaging to the fibres of the rug. The sediment is very fine and if there is too much dry sediment in the rug it starts to act like a knife and cut the fibers creating wear and tear of the pile.
Just like carpets and upholstery, rugs act like a filter and collect all the pollutants in the pile. Just like any other filter it needs to be regularly cleaned.
It is not a bad thing that carpets and rugs collect soil, sediment and pollutants, in fact it is very healthy. By collecting all the pollutants like a filter, it stops small particles being airborne, and so you do not breathe them in helping to reduce such problems as Asthma and Allergies.