If you don’t know what a sitemap is, or have never created one…read on. A sitemap is a list of the individual pages on your website displayed in a hierarchical fashion similar to a table of contents, or index. They are sometimes used as a planning tool during the developmental stages of a site design, but more importantly, sitemaps act as a powerful navigational aid by providing a site overview at a glance. Sitemaps also benefit search engine optimization by ensuring that all the pages of a site can be found by web bots.
At one time, sitemaps were viewed as a luxury, or at the very least, not vital. For new sites, they are especially critical as it can take several months for a new site to get crawled and indexed by the search engines. Implementing a sitemap and submitting it to search engines and web analytic utilities such as Google Web Master Tools, will greatly aid in the indexing of your site. Sitemaps do not guarantee all links will be crawled, and being crawled does not guarantee indexing. However, a Sitemap is still the best insurance for getting a search engine to learn about your entire site.
If your site is very large, has a complicated navigation system, or employs Flash or JavaScript menus that do not include html links, parts of the site may never get indexed. Even if you only have a small site, having a sitemap will ensure that all your pages are linked to and ensure that they will be picked up by the crawlers.
Users and crawlers will now be able to access deep links and nested pages much more readily. Having well named, SEO friendly urls in your sitemaps creates the added functionality for users to conduct site-wide searches of the sitemap for specific keywords that they may be looking for in the site. Sitemaps have also been shown to increase PageRank and link popularity to all the pages it links to. While it is more important to have high quality links pointing to your site, you should not underestimate the usefulness of internal links pointing to your own pages.
Sitemaps are written and saved as an .xml file which is the document structure and encoding standard used for webcrawlers to find and parse sitemaps. As such they are very unforgiving and must contain only valid XML syntax. (http://validator.w3.org/ ) Sites are able to be prioritized on an sliding scale from 0.1 to 1.0. Sitemaps are also beneficial in letting search engine bots know when you last updated your website.
Even after reading this post you are still not convinced of the benefits of a sitemap, remember that Google has stated that a sitemap is a ranking factor for your site. Although it may be a small one, added together with several other smaller ranking factors, they all add up to substantial ranking factors and is considered the best practice for any website.
For further information, check out this page in the Google Webmaster Tools Help.