A sitemap is useful to let Google discover all the pages of your site. Even though it's not necessary to submit a sitemap if you use internal links properly, sometimes it's just to easy to obtain one. If you have a blog, your feed could be submitted as a partial sitemap. If you have a simple site, ROR Sitemap Generator will crawl it and generate a sitemap.
The sitemap protocol, developed by Google, and supported by Yahoo and Microsoft, is useful to create complex sitemaps that include information about the last update of a page or its importance. "Sitemaps enhance the current model of Web crawling by allowing webmasters to list all their Web pages to improve comprehensiveness, notify search engines of changes or new pages to help freshness, and identify unchanged pages to prevent unnecessary crawling and save bandwidth."
Vanessa Fox recommends to create a sitemap especially for new sites and dynamic sites with a lot inaccessible pages.
Related:
Webzari - visual site explorer
How to create a good site
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