The Sitemap.xml can be seen as a list of important URLs. Its intention is to show Google and other search engines which sites should be crawled in any case. It is recommended to use the Sitemap.xml as an additional aid for better crawling results.
While the Sitemap.xml has a mean to show search engines which pages are important, visitors do not search for pages in the Sitemap.xml file. Website users navigate their way through the website structure until they find the expected information. That's why it is important that important pages are not only listed in your Sitemap.xml file but also have a suitable amount of incoming links to make it easier for the visitor to find the page.
So pages in the Sitemap.xml without links can be found by search engine but not by users.
Here is an interesting link about Common SEO Mistakes and How You Can Fix Them.