Sitemaps are important in the way that they help Search Engines by telling them about the prescence of pages that they would otherwise overlook. Or due to the bad architecture of the site would not find. In a sitemap all the URLs are in one place and they can easily be found
There are two types of site map. HTML sitemap and XML sitemap. HTML sitemap is only for website visitors so that they can find all pages from one place of that website if the site is large. XML sitemap is used by search engine spiders for better indexing of web pages. It is more important for website indexing.