Google™ uses links to measure the importance or popularity of web pages. This is generally well understood but if you're confused then our what is link building blog post will help.
That aside, Google places important web pages, those with a good link profile, at the top of the search results so it should be clear that having links to your pages helps them get found. In simple terms, popular content, pages with links, rise to the top of search results. Enter the link sellers.