There are many tips and tricks that help in optimizing a Website but one of those, the importance of which is sometimes underestimated is sitemaps.
Sitemaps, as the name implies, are simply a map of your site.
Using sitemaps has many benefits, not only easier navigation and better visibility by search engines. Sitemaps offer the opportunity to inform search engines immediately about any changes on your site. Of course, you cannot expect that search engines will rush right away to index your changed pages but certainly the changes will be indexed faster, compared to when you don't have a sitemap.
Typically on one single page you show the structure of your site, its sections, the links between them, etc.
Sitemaps make navigating your site easier and having an updated sitemap on your site is good both for your users and for search engines.
Sitemaps communicate with search engines. Your site map you tell search engines where you'd like them to go and what website pages exist on your site.
Sitemaps are one of best Website design practices and with the adoption of sitemaps by search engines, now they become even more important. However, it is necessary to make a clarification that if you are interested in sitemaps mainly from a SEO point of view, you can't go on with the conventional sitemap only (though currently Yahoo! and MSN still keep to the standard html format). For instance, Google Sitemaps uses a special (XML) format that is different from the ordinary html sitemap for human visitors.
If you are asking why two sitemaps are necessary. The answer is obvious - one is for humans, the other is for spiders (for now mainly Googlebot but it is reasonable to expect that other crawlers will join the club shortly). In that relation it is necessary to clarify that having two sitemaps is not regarded as duplicate content.
Take a look at Fast Marketing Plan's Site Map at http://www.FastMarketingPlan.com/sitemap.htm
Budget: $25 - $200 monthly