The basic purpose of sitemap is to give robots a machine readable list of your site's pages that you want to have crawled and indexed. If you have no sitemap your site can still be crawled and indexed but it may take longer for all your pages to be found.
:: dashing off to foo because this has set off a line of thought ::
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