Keep in mind what Google actually "sells".
Where Yahoo! was first to market, had more money and way more customers, they committed one egregious sin... they blended in paid advertisers into search results... in other words, they put the interests of the advertiser first.
Google however, despite many perceptions, actually tried to put the customer first.. Meaning, they made their search results more relevant to the question, regardless of which advertiser was trying to "buy their way in".
Google sells "relevance".... so.... why do you care?
If you do the work for Google, making your site and your pages more relevant to the keywords and making discovery easier, Google will reward you with top search engine ranking.
In the case of the sitemap there are two targets. Google will reward your site by posting all of your pages in XML form which is very easy for Google to read, index, sort, alphabetize.... whatever it wants to do with it. If you didn't upload this, Google would have to "read" your website complete with all of the disorganized pictures, images, broken tags, etc. You just made Google's job easier so you get a reward (in page rank).
You also get a bonus for "internal links" which means pages that link to each other within the same website. The sitemap give Google a hint as to which pages may link from one to the other (particularly from this sitemapSEO.aspx url).
Let me know if you have any other questions about how to leverage your sitemap, but for certain Google for "Google Webmaster Tools" and upload your sitemap or you're giving away money.
This article may help you that I'll be presenting at SQL Saturday in Florida: A website is not a funnel... it's a spiderweb (http://blog.nielsendata.com/post/Mind-Tricks-For-Business-A-Website-is-a-Spiderweb-not-a-Funnel-Jared-Nielsen-FUZION.aspx)
Hope that helps,