As defined at sitemaps.org “Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling”.  A sitemap is an XML document that typically is placed at the root level.  The document contains information on the site structure and contents for search engines.

Normally, whether or not you have a sitemap, a search engine discovers the contents of your website by following links.  Those links can be on outside websites, or they can be on your own website.  As long as there are links pointed to a page and your website has good content odds are very good that a search engine will quickly discover the page all on their own.

This brings up the question of whether or not a sitemap is necessary.  If a search engine will find everything without one why bother.  My personal opinion is that if a sitemap can be created automatically, like through a plugin or module, then go for it.  However if you have to create one manually I have a hard time justifying that much work for something of negligable use.

The reason I belive that is because if your site is structured in a way that a sitemap is necessary for search engines to find their way around you have already failed.  One of the core foundations of a solid SEO strategy is to have an easy to navigate website.  Since search engines navigate the same way as surfers if a search engine can’t find it, neither can your customers.

Adding a sitemap to a good website will likely produce no results at all.  Adding a sitemap to a poor website may alert search engines to content they couldn’t find before.  However that page is highly unlikely to ever show up in search results because it would lack incomming links.  I have yet to see a study that showed that the presence of a sitemap resulted in any SEO advantages that wouldn’t be better fixed in other ways.