We all need Maps for navigation similarly the bots which crawl your website also need a navigator which tells them the where abouts of the infomation stored on your website
Google Sitemaps Beta is a service by Google designed for website owners which enables spiders to proactively and automatically crawl information about your website to Google. This sitemap can help provide Google with more information about your website than usually crawled by the Google.If your site has dynamic content or pages that aren’t easily discovered by following links, you can use a Sitemap file to provide information about the pages on your site. This helps the spiders know what URLs are available on your site and about how often they change.
A Sitemap provides an additional view into your site. This program does not replace the normal methods adopted by Google of crawling the web. Google still searches and indexes your sites the same way it has done in the past whether or not you use this program. A Sitemap simply gives Google additional information that the search engines may not otherwise discover.
You can create Google Sitemap within a span of few minutes using the tool Google Sitemap Creator.