Basically, a Sitemap provides a listing of pages on a website. By creating a Sitemap and submitting it to Google, the webmaster ensures that the search engine will have all the needed details about a site. That way, Google will be able to find URLs that cannot be found when Googlebot crawls the web.
A Sitemap is also a practical reference to Google for providing metadata about certain kinds of content, including images, news, mobile and video. For instance, inputting data regarding an image can alert Google as to the topic of the photo, license, and type. A Sitemap also is used to indicate when the content on a site is changed or how often it will be altered.
A Sitemap is especially useful for sites that feature images that may be hard to find by Googlebot or contain pages that aren’t linked or have few included links. Sites that contain compelling information or content can also benefit by the creation of a Sitemap. As a result, the information provided by a Sitemap enables Google to assess the framework of a site and better crawl a site