When contemplating carpet cleaning, most people will only consider the aesthetics or appearance factor, but there are also other important health and financial issues to be borne in mind.
Soiling on your carpet is obviously not good for several reasons, as outlined below.
Soiling diminishes the beauty of the carpet.
It harbors insects, microbes and pathogens.
And some types of soils cause wear and tear.
Fortunately, almost all carpeting being sold today, comes with a soil and stain resistant finish that repels soiling, but this obviously does not guarantee that your carpet will remain clean indefinitely.
The finish will wear off with time, or with treatment by harsh chemicals, and will need to be replaced eventually.
It is also not effective for all types of stains, and often, the soiling just lodges on the surface or between the carpet fibers.
There are basically three types of soil.

1. Water Soluble - Usually consisting of soluble sugars, starches, and other food residues.
   
2. Organic or Inorganic Solvent Soluble - Like oils, tar, grease, make-up, paint, etc.
   
3. Insoluble - Sand, quartz, pollen, hair, mites, etc.
   

Most of the soil in your carpet will be insoluble, and this will cause the most physical damage.
This type of soil is usually hard and abrasive, and with wearing, will cause the carpet fibers to be scratched and chafed.
Over time, the light reflecting properties of the carpet fibers will change, and this will cause it to look worn.
The other kinds of soil also cause damage if not removed quickly.
Water soluble soils from food or drink spills are usually acid in nature, and may react with the dyes in the carpet and cause the color to fade.
Solvent soluble soils will cause yellowing as they react with the oxygen in the air and generate resinous by-products.
Insects and mites cause damage by sometimes feeding on the carpet fibers, but mostly by excreting products that are harmful.
Bacteria also multiply as they feed on the insect excrement, and they themselves carry or generate pathogens that are deleterious to your health.
Your carpet is indeed a filter.
In fact I believe it is the most efficient filter in the home.
It collects everything, from particulates tracked into the home on the soles of busy feet, to dust, pollen, and atmospheric pollution in the interior environment.
Sometimes the existence of soiling is not apparent.
The carpet may appear to be clean, because it is not showing up any ugly stains.
Don't get complacent.
Unseen pollutants may still be steadily and actively causing damage to your carpet, and harm to your health.
Vacuum.
Often.
Clean up spots, spills, and stains immediately you become aware of them.
Call in a professional carpet cleaner to deep clean your carpet at least once a year.
Carpet manufacturers actually recommend a deep steam carpet cleaning every 18 to 24 months, but it is certainly better to do it more often if you can.
A word about vacuuming here.
"Often" means at least once per week, but if you can do it every day, that's great! It is particularly important to vacuum daily if you have kids, pets or heavy traffic in the home.
Also, maintain your vacuum properly, according to the manufacturer's operating instructions.
Above all, do not allow the dust collector to become overloaded.
By consistently following these tips, you will avoid damage to your carpet by soiling, and extend its lifetime considerably.