Mold, bacteria and chemicals of all kinds can settle or be tracked onto carpeting. These biological microbes are alive and thriving in your carpets, and if it remains dirty, they will continue to breed and grow.
Sticky residue from conventional shampooing methods will cause rapid re-soiling due to lack of rinsing and extraction of the chemicals used. Spots and stains will resurface because they were only pushed a little deeper, and the traffic areas will appear dirty sooner for the same reason.
Sand, dust and other hard particles that get embedded in your carpet will cause abrasion of the fibers, making the carpet look dull. Light no longer reflects properly as the tips of the carpet tufts plume, causing a shadow effect – permanently.
All of these things can be prevented by annual professional steam cleaning!