If a website is your storefront, the Sitemap is the the foundation and marketing office combined. A Sitemap is simply a page within your website that provides information to search engines about your site. That information is presented in such a way as to optimize the quality of the search engine results the engine will produce. Search engines are the heart of a company's online presence and a Sitemap should be standard protocol when building a company website.
A Sitemap is a great way to update search engine results too, notifying them of changes made to your site. Pages that have content that doesn't contain HTML code or is dynamic, such as those with flash coding embedded in them, are not crawled by search engine spiders. Dynamic pages mask the code that search engine spiders look for when crawling a site. With a Sitemap however, any pages with this kind of content are made available to the spiders and can be indexed. Sites with a large amount of pages likely won't have all of them crawled unless they are listed on the Sitemap.
Sitemaps are important for webmasters because they maximize exposure of all pages to search engines and maximize the likelihood that the pages will all be indexed. Indexed pages are those that appear when you use a search engine. The higher pages appear in the results, the more likely people are to follow the listing to the site. Think about how you use search engines. If you are like most people, you will read through the first and maybe second page of results and not the rest. This is especially true when people are interested in making a purchase, as they are already motivated to buy and are programmed for quick gratification.
XML Sitemaps, those that are programmed in extensible markup language rather than traditional HTML, are more commonly used in recent years. XML Sitemaps cannot be submitted if they contain errors, thus enforcing the standards used by search engine crawlers. XML Sitemaps also include metadata in their structure, which changes information within the Sitemap if the site is changed in any way. Change is good in this respect because search engines prefer sites that have content that is updated often. Sites that update often are rewarded by gaining increased weight in terms of indexing.