The sitemap.xml (https://de.onpage.org/wiki/Sitemap) can be seen as a kind of list of important URLs, with the intention to show Google and other search engines which pages should be crawled in all cases. It is recommended to use the sitemap.xml as additional support for better crawling results.
While the sitemap is there to show the search engine which pages are important, visitors do not search for pages in the sitemap.xml. Website visitors navigate their way through the website structure until they find the information they are looking for.
Therefore it is important that these important pages are not only listed in the sitemap, but that they also have enough incoming links to make it easier for users to find them.
Pages in the sitemap.xml without links can be found by search engines, but not by the user.