7 BIG advantages to submitting an XML sitemap ...even when your site is already using an HTML sitemap

As you know, an HTML sitemap is a single webpage that is essentially a list of links to all of your site's web pages. It provides a means by which a searchengine spider can easily find and crawl all of your site's pages. On the other hand, a XML sitemap (aka a Google Sitemap, although it's used by Yahoo and Microsoft as well) is a special file that provides searchengines with specific directives about what pages to crawl and how often.

Searchengines are not required to strictly obey these directives but they do tend to use them as guidelines.
Every site needs an HTML sitemap. An XML sitemap, on the other hand, is not strictly necessary. But there are several advantages to having one...

You increase the chances that search engines will find all URLs you want indexed. You can set a priority and last modification date on your more important pages to ensure more regular crawling. Your sitemap data is integrated with your other site data in Google Webmaster Central, making these associated tools even more useful.

Google states that XML sitemaps are particularly useful if...Your site has dynamic content.
Your site has pages that aren't easily discovered during the crawl process, such as pages featuring AJAX or Flash.

Your site is new or isn't well linked. Google crawls the web and finds pages by following links from one page to another. If your site isn't well linked, it may be hard for Google to find and index all of your pages. Your site has a large number of pages that aren't well linked to eachother, or are not linked to at all.

While an HTML site is a must-have, an XML sitemap is nice-to-have—but not necessary. However, if your site has pages that are difficult to get or keep indexed, an XML sitemap is the best way to keep search engines instantly and continuously informed of changes to your web site while telling it specifically which pages you would like to be given priority in the indexing process.