Many of the clients I talk with have an understanding that a Google sitemap is a way to positively impact your natural rankings. This of course is not true, the Google sitemap merely allows you to tell Google that you have the following web pages within your domain and that you would like them to be crawled. You have the ability to set how frequently the page is updated: Daily, weekly, or monthly. The last option you have within the Google sitemap is the option to set the priority of the page.
This is the webmasters way of telling Google which pages are more important than others. For example the home page would be more important than page to showcase links so it would receive a higher priority. Google will then access the Google sitemap you have uploaded and see the listings of pages and information you have provided. This will result in Google being able to find and index those pages faster and get a basic understanding of the structure of the website. This is not to say that Google wouldn’t find the pages of the website on there own, but it facilitates Google finding them faster.