What is a sitemap?
A sitemap is a simple file that contains details of some or all of the pages on a website. A sitemap helps search engines determine quickly what new pages might need to be examined for content and further consideration for search traffic.
If there is no sitemap, search engines must follow menus and other links inside a website to determine what content has been changed and if there are any new pages. This is significantly more time-c0nsuming for a search engine to perform, and is therefore often not done.
Sitemaps help your website get indexed more often and in a more controlled fashion than relying on search engines to do alll the legwork.