The primary reason for submitting a site map is to tell Google and other SE's all about your websites pages and making it easier for SE's to index them.
quicker discovery of pages, and hints to the search engines of the importance and frequency of changes to pages. Don't forget you can submit a sitemap to more than just google.
Sitemaps are very important as they inform search engines about your site content so search engines can easily scan and index your pages. Focus on getting into Google first by submitting your sitemap via Google Webmaster Tools... when your site is indexed in Google and has many inbound links from other sites, it will be an easier job and other search engines will find you automatically...
Google found all my pages without a sitemap, but if they just seem to be ignoring certain pages for some reason... you can slap it into the sitemap and Google has a better chance of picking it up.