The XML Sitemap contains information that helps the search engines get through your site. It is simply an XML file that contains the information for the URL’s that you have on your website. It can simply a list of the URL’s, or can contain more in depth information such as page priority and the last modified date and time of a page.
XML Sitemaps have the benefit of being beneficial to your website and being very easy to build. They can be more complex, but the only mandatory element is that you have to include a URL that you want the Search Engines to look through and index.
With an XML Site map, another aspect is that you are able to show how often a certain page should be modified. This however, does not mean that everytime it is modified that it will be searched through again by the search engines.
Defining the priority of a page on your site does not have an advantage in the search engine rankings. It just ranks the pages in priority order from the other pages in the website.