Other than for aesthetic purposes, did you know that professional carpet & upholstery cleaning will improve their longevity? If allowed to remain for extended periods of time, abrasive soil accumulation will scratch and permanently damage fibers, causing them to appear dingy and 'uglied-out'. Unfortunately, damaged fibers can not be restored to like-new conditions, however, the high-end professional has the ability to at least blend damaged areas to make them appear the best that they possibly can. Preferably, regular professional cleaning is the only way to prevent this phenom from occurring in the first place.
Concerning furniture upholstery, things are bit different because it tends to attract body oils and sweat as apposed to abrasive debris. However, contaminants from clothing can attach themselves to upholstery as well.