Having an accurate, properly generated and regularly submitted search engine sitemap to google / yahoo and bing is vital in the process of indexing your site. This process consists of search engine spiders crawling throughout your entire site, actively following any links, regardless of where they lead.
Some pages or files on your website may not be reachable via normal methods, so listing them in a sitemap will make sure they are found. Within search engine sitemaps, you can also list the frequency of which certain pages are updated to in effect, ask search engines to return at a later date to re-index the site, in case any changes have been made.
The sitemap is a must have for any web site. Rob Ellis Web Design has the tools to generate proper sitemaps for your company.