Sitemaps are important.  Its gives both the human and the machine (spider) a definately place to understand how the site is structured.  This is the main reason it is so important.... the structure.

Sitemaps reeally come in 2 forms; those for the human in HTML and those for the spider in the form of XML.

HTML sitemaps are to allow your vistors an easy way to jump to a page with in your site.  You can follow 2bebrave's sitemap by clicking here sitemap

XML sitemaps are structure in a code like mannor so that the search spider can understand how the site is laid out.  The removes ambiguity of the spider making a mistake or missing pages.  You can see an example of 2bebraves XML sitemap in the image below.