While it is always an option to clean your own carpet with kits or rented equipment that will rarely have the same affect as letting professionals do it for you. They do this for a living and use equipment that is rarely available to the general public. While perhaps not the biggest difference, it is a major one.
A good carpet cleaner knows how and when to use every single piece hosing, and every attachment in his arsenal to its fullest affect, and will employ them to great results. Large boilers and pumps mounted on the sides of trucks power their Carpet Steaming equipment. Attachments called wands are used often, and to great affect by professionals, during Hot Water Extraction. Wands clean and help preserve the carpet, where If used improperly, they will damage the carpet itself beyond repair.
A professional should always keep his equipment in top form, but “should” is not “do” and damaged or malfunctioning machinery could much diminish the affect of the cleaning. You should always to ask when the machinery was last repaired, or what kind of condition it is in right now.