Why should you
Many people will wait and wait before cleaning their carpet because it doesn't look soiled. And it doesn't. It looks pretty darn clean after 2-3-5- even 10 years! It doesn't get walked on a lot, gets vacuumed regularly, its doing pretty good. So does it need cleaning?
In some cases, no, it really doesn't. If its in a spare room that hardly gets used a good vacuuming should suffice. However, if its a room that is used regularly but gets vacuumed a lot, it may not appear like it needs cleaning when in fact it is hiding a lot of soil.
Carpets are mostly made of nylon, basically a clear plastic fibre, like fishing line, but as fine as a hair. The first generation of carpets used simple round fibres. They discovered the round surface would magnify the soil making it appear more soiled than it actually was.
To compensate for this today's modern carpets are designed to hide soil. They modified the shape of the fibers, making them tri-lobed (like a clover leaf) or square, or by creating holes through the length of the fiber during the extrusion process. This causes the light from the fibers to diffuse rather than magnify, thus making a carpet look cleaner than it actually is. So by the time a new carpet looks soiled, it really needs a good cleaning.
80% of soil in a carpet
As you can see in the electron microscope picture the soil sticks to the fibre much like a scab on skin or a barnacle on a rope. It won't vacuum off but needs to be cleaned off. If left you can imagine how this soil will wear at the fibres much like barnacled ropes would cut through each other. This is where proper steam cleaning comes in. The process will wash away those soil particles leaving the fibres clean.