If you have a static site with twenty pages that doesn't get new pages added very often then yes, a site map probably isn't of a whole lot of use if your website has good architecture.
However, if your site is 30,000 pages and gets new content added regularly, then an xml sitemap is useful to make sure that the engines know about all of your pages.
Using multiple sitemaps can be useful to help you diagnose what type of content Google is crawling best. A hypothetical example is that you have a large site where you a) sell baking supplies b) have recipes and c) have user profiles that you want indexed. You could submit a site map for each area (then a master sitemap that lists each of the sub sitemaps).
In Google Webmaster Tools, you get a report that says how many pages you submitted for each site map, and how many of those pages are indexed. using the above setup, you might find something like:
baking supplies has 50 URLs indexed out of 2000 submitted
recipes has 10,000 URLs indexed out of 11,000 submitted
users has 500 URLs indexed out of 1000 submitted
At a glance, you can tell that something is up with the products you're trying to sell and that Google isn't indexing that section very well, and you know to focus on that section, and maybe there's a bug in the code that put a noindex on most of the pages on accident.
Does that help?