Why do Google kick the Low Quality Website?

Google is modifying its search system. They try to stem the existence of a site that is considered low quality, appears in the row above the search results. But these changes are still only valid in the territory of the United States alone.
Quoted from Sify, Monday (28/02/2011), Google is changing the mathematical formula to get rid of sites that offer low value. It’s like some sites that just provide a little content and instead take the user to a page filled with ads.
This search engine giant before it was intended to dam sites that simply mimic content from another website or present information that is not useful. And, this was it.
Google explained, this change lowered the website with low quality and raise the quality website rank high in search results. Web site has a condition such as high-quality original content and have had a profound analysis.
Google is working on projects such change since about a year ago. Its implementation started in the United States and the plan would be visited other areas.