Sitemaps is good for letting you know how google is seeing your site. It will tell you of any errors that could be occurring, the crawl status of your site, the anchor text for backlinks to your site, higher ranking search terms for your site, etc..
How critical Google Site map is depends on how dynamic your site is and how deep Google is able to index that site. If your site is quit large, made with a CMS (like most large sites) and the way the links are made in this CMS, makes it hard for Google to index your site, a Google Site Map can remedy this problem. If, how ever your site is a static html site and Google has indexed all of the content, I donít think a Google Sitemap will make much of a difference. It will also not hurt your site.
And most important things I've got from Google sitemap is: 1. My site position for 20 keywords in Google SE query. 2. Top 20 keywords that drive visitor to my site plus their average position in the query result.