The primary advantage of the hot water extraction cleaning method is that effective cleaning is possible using only hot water, or hot water with mild biodegradable solutions. This avoids the problems associated with detergent residues that can remain in the carpet with other cleaning methods. Detergent residues on carpet fibers can attract dirt from the soles of shoes as people walk on a carpet, causing the carpet to become dirty again soon after cleaning." For more in depth information and full details on the benefits of this technique see the rest of the Wikki here.
"Carpet Cleaning." Wikipedia. Wikimedia Foundation, 22 Oct. 2012. Web. 07 Nov. 2012. <http://en.wikipedia.org/wiki/Carpet_cleaning>.