Eliminate the need to rent a machine
This is probably the biggest reason to hire a professional rather than go at it yourself. When you add up the cost of the carpet cleaning machine, the cost of the cleaning chemicals and the additional tools you may need to do stairs or upholstery you are not saving all that much and the rental or homeowner cleaners are no where near as powerful as professional carpet cleaning systems.
Different types of carpet
There are many different types of carpet on the market, nylon, berber, wool and so on. Professionals have the skills and knowledge to determine what type of carpet you have and what type of cleaning agents will work best without damaging the carpet.
A true professional will be using powerful truck mounted equipment capable of producing extreme heat, the heat that is needed to kill germs, dust mites and other contaminates found deep in the carpet pile.
Difficult spots and stains
There are many different types of stains, some can be very challenging such as urine, red dyes found in kool-aid. These types of specialty spotters are just not available to the public.
The physical aspect
Carpet cleaning is very strenuous work, just lugging the machine to and from the store is enough to cause back pain and the actual cleaning is even harder work.
The portable equipment available for sale or rent is with out a doubt leaving dirt and soil behind. The vacuum motors in this type of equipment are no comparison to the what the pros use. Not only is dirt being left behind but usually too much water and cleaning agents are left behind as well. Over wetting can cause mold, mildew and odors. Leaving chemicals behind will cause faster re soiling from sticky residue.
Leases and security deposits
Most leases state that the carpets must be cleaned by a professional carpet cleaner, and they will ask for a receipt to prove it. They know that a professional job is far superior to the do it your self jobs.