I agree, a Site map helps insure GoogleBOT finds its way through your site without giving up and going elsewhere.
The better it indexes your site the better it "should" reflect on your Page Rank but as a combination with MANY other factors
Mekhu, no, those are called Site Links and are determined by Google based on how much of an authority they think your website is for that keyword / phrase.. This is most commonly seen for company names since they most likely are the most authoritative site on their name..
But you can also see it for other more generic terms, but it's harder, much harder..
The sitemaps, as said earlier, are really there just to help Google find all of your pages.. But, if your internal navigation is clean and effective, they aren't really all that useful..
It does not hurt to submit a sitemap however and it may even speed up indexing of some pages, but it is not required if, as Jaan said, you understand how to link to your content so that both visitors and search engines can find it all easily.
Actually the creation of navigation schemes which are helpful to both humans and bots should be taught in SEO 101.