The penalty is part of Google’s larger, ongoing effort to reward high-quality sites, especially those that regularly produce relevant and informative content. Google lumps these sites under the term “white hat” SEO. Google’s nemesis is “black hat Web spam,” in which sites use SEO techniques like keyword stuffing and link schemes to rank pages higher than they should.
As part of the over-optimization penalty, sites that engage in black hat tactics will receive decreased rankings. Who wants to get on Google’s bad side?