Site map is the thing that let's Google bot know about changes or updates on your website. Google bot always checks this out and webmasters submits sitemap in webmaster tools for better indexing. Hope this helps.
First you can watch out the previous thread of sitemap - http://forums.digitalpoint.com/showthread.php?t=1978795 Sitemap is actually a map of your whole site. It's a tool that makes the visitors to navigate through pages very easy and comfortably. There is no other options to find any particular page within a website. Besides that a sitemap is really important for search engines as well as search engine optimization.
Sitemap importance::-- 1. Help with canonical URLs 2. The Last modification date field in the sitemap file can aid Google in quickly locating the actual change in the page
The sitemap helps your pages get indexed faster when you submit your sites sitemap on google webmaster tools