Google's algorithms use those keywords to understand a page better. Sometimes a page ranks well for some keywords that are not even contained in that page, but they're used in links from other pages. For example, Yahoo.com is the fourth result for [under 18] mostly because of backlinks that use this anchor text.
"This information is useful, because it helps you know what others think your site is about. How sites link to you has an impact on your traffic from those links, because it describes your site to potential visitors," notes Vanessa Fox.