You don't see the cleaning benefit as you don't have a deep enough understand of the science and process of carpet cleaning, ( I know that sounds really condescending )
The big 'myth' people have of carpet cleaning is once you get your carpet cleaned they get dirty quicker, this can be partly true if you are leaving a heavy residue in the carpet to attatch to dry soil. If you can flush enough pure water through the carpet it will remove the prespray and emulsified soil leaving nothing behind.
Most carpet cleaners flush the carpet with a detergent solution that leaves ..... detergent in the carpet.
Given the choice would you prefer me to clean your carpets and leave nothing or would you prefer me to leave a cocktail of upto 20 different chemicals?