Having an XML sitemap for your site is an integral part of search engine optimization (SEO).
By creating and submitting XML sitemaps you are more likely to get better freshness and coverage in search engines.
A sitemap is a way of organizing a website, identifying the URLs and the data under each section. Previously, the sitemaps were primarily geared for the users of the website. However, Google's XML format was designed for the search engines, allowing them to find the data faster and more efficiently.

Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site.
Web crawlers usually discover pages from links within the site and from other sites. Sitemaps supplement this data to allow crawlers that support Sitemaps to pick up all URLs in the Sitemap and learn about those URLs using the associated metadata. Using the Sitemap protocol does not guarantee that web pages are included in search engines, but provides hints for web crawlers to do a better job of crawling your site.

Here are 4 simple steps to get it done
1. Enter your full website URL and some optional parameters in the form below.
2. Press 'Start' button and wait until the site is completely crawled (the progress will be indicated)
3. You will see the generated sitemap details page, including number of pages, broken links list, XML file content and link to a compressed sitemap. Download the sitemap file using this link and put it into the "public_html/" folder of your site.
4. Go to your Google Webmaster account and add your sitemap URL.
Please check About Sitemaps for more details.