In default the Google gets to crawl all your links in your website, especially even the server links. Maybe you try and learn, if you just update your site to Google webmaster without sitemap.xml, even your sites server path will be shown as a link in SERP. Especially Apache root address which can make your site vulnerable to hackers.
So understand sitemap.xml is like, guiding your Google Crawler to crawl only certain links. You can generate sitemap for free. Search for Generate free site map, a site map will be generated. Then open the site map, in that adjust values, for which links the crawler should give importance. Like the index page and blog page, the value will be from 0.10 to 1.00 given under each link generated in sitemap.xml. Check that if sitemap.xml contains every links inside your site, if not add it. Once you're done with the site map, upload it to the your server database and then upload it to Google webmaster.