This is a great video that shows how important carpet protector is to your home. When your carpet is made it has a layer of carpet protectant put on in the final stages. This chemical breaks down over time and allows stains to "stick" to the fibers. Professional cleaning and proper re-application of carpet protectant can extend the life of your carpet and keep those nasty stains from showing up.
* Note how the liquid attacks the carpet. It starts in a small fiber cluster, soaks through and then penetrates the rest of the carpet and through to the carpet backing and pad. By keeping up to date on your carpet protectant applications you can keep those fiber clusters from allowing stains to seep through and clean up spill without staining issues.