This plugin provides Search Engine Optimization (SEO) for Title, Description, Keywords as well as customized additional headers for SE standard pages. It also support XML sitemap file building for your site contents including core site map menu urls, user profile urls, and popular plugin page view urls via site map channel API. Easy submit sitemap to major search engine like Google, Bing thru Update Notification (ping) etc..
You can learn more about the important of having SEO / Sitemap on your SocialEngine website by reading this guess post: Giving the Google Spider A Map To Crawl
This plugin has interface to add custom channels for 3rd party plugins very easily. This feature makes SEO / Sitemap a very flexiable, extendable, and customizable plugin. It comes built-in with support for all SE official plugins, as well as popular Radcodes plugins.
NOTE: The plugin does not make any guarantee on the search ranking placement of your site. The ranking position is up to the search engine, which no one really have control over (unless you pay to be on top etc..) For the Sitemap part, the main job of the plugin is to "generate the sitemap" of links on your site for you, so that you do not have to do it manually (imagining create a sitemap with thousand of urls yourself). It also (optionally) help you submit that sitemap to the search engines, so that you do not have to manually do it. Submitting a site map does NOT guarantee that it would have good or bad effect on your site placement in anyway. Sitemap XML helps search engine bots find pages on the site. If you have question about placement / ranking position of your site, you should contact search engine or SEO firm for full analysis regarding using title, description, keywords that work best for your site content.
Below is the direct quote from WikiPedia, you can learn more about site map on Sitemap Wiki page.
Google introduced Google Sitemaps so web developers can publish lists of links from across their sites. The basic premise is that some sites have a large number of dynamic pages that are only available through the use of forms and user entries. The Sitemap files can then be used to indicate to a web crawler how such pages can be found. Bing, Google, Yahoo and Ask now jointly support the Sitemaps protocol.
Since Bing, Yahoo, Ask, and Google use the same protocol, having a Sitemap lets the four biggest search engines have the updated page information. Sitemaps do not guarantee all links will be crawled, and being crawled does not guarantee indexing. However, a Sitemap is still the best insurance for getting a search engine to learn about your entire site.