Google ranks the websites based on the importance of the content or service provided by the site. Depending on the amount of useful information a website has, Google even provide inline search box for each item within the Google search result.
The inline search box will help the user to search within the specified website directly from the Google search result. This particular facility can be viewed as layman version of the advanced Google search queries. Most of the Google search experts are very much familiar with the site: keyword used to restrict the Google search for a specific website. Now Google packaged the same thing for common usage, depending on the importance of website content.
Must read: Google knows themself better than other image search engines