From a Search Engine Optimization (SEO) point of view sitemaps are extremely important. First we would like to clearly state that adding a sitemap does not affect search rankings. However, if there is some page on your site that is not indexed, then sitemap provide you a way to let search engines know about that page, so it can be indexed properly.
Sitemaps are very useful for new websites and blogs. As most new blogs usually don’t get many backlinks to their individual posts, it becomes harder for search engines to discover all of their posts and pages. Sitemaps allow those websites to help search engines crawl their sites more efficiently.
For popular websites, sitemaps allow your website to interact with search engines more effectively. Search engines can guess how often your site is updated, which parts of your website are more frequently updated than others, set crawl rate for your site, etc. This improves the overall visibility of your site to search engines.