Nice explanation in the piece from Bret Fausett, who publishes Lextext.com.
When a domain expires and changes hands, Mr. Fausett said, Google can now more easily find, scan and index the new site, so it does not mistakenly point searchers to a site with irrelevant content, or place advertisers on sites with content that does not match their products or services.
That alone could profoundly affect the domain name market, which has rebounded partly because of another Google service, AdSense. Through AdSense, Google pays publishers to display text ads related to a site’s content. Speculators often buy the expiring domains of even marginally popular Web sites and replace the site’s content. But because the practice diminishes the usefulness of Google’s search engine, the company has long sought ways to curb it.