Hi David, For SEO to be effective it is important that the search engine spiders and web crawlers can properly index a site. Based on your site's architecture and its links (both external and internal) the engines can establish and weight the profile of how each interacts to map out which pages have the highest significance to their index.
One of the most effective ways of encouraging full and extensive visiting and indexing of your site by the spiders is to integrate a site map (or maps) into your development. Sitemaps are an easy way for web masters to represent the structure of a site and inform search engines about pages on their sites that are available for crawling. Human visitors also appreciate site maps as navigation.
An HTML sitemap is an actual page on a site containing links to the website's most important pages. Any site with anything approaching complex navigation should consider an HTML sitemap for visitors. Whilst an HTML sitemap can be read and the links followed by spiders (so some passing of link juice is involved that might affect SEO), it also has the advantage of being readable by humans as well.
A visitor might refer to an HTML site map to help them navigate when they can't find a specific page easily. This visitor readability and usability issue that allows users to locate pages within a large site will give weight to your site in the SERP's, (search engine ranking pages) as they will weight your pages favourably for catering to your users and being user-friendly.
HTML sitemaps, however offer only a general guide to a site, an overview, not necessarily containing all the links available to spider in a website. This is especially the case for websites that contain dynamically generated pages (shopping carts, etc).
editor's note: for complete post, see http://www.seoconsult.com/on-page-search-engine-optimisation/what-effect-do-xml-and-html-sitemaps-have-in-relation-to-seo.html
I hope that you r query has been solved.