XML Sitemap, as it is clear from the name it is a kind of a map of your website written in XML language. This is made for robots, crawlers of search engines so that they can crawl your website with very ease and each of your URL can get indexed in search engines.
XML is easy for bots to read so they don't need to crawl your website and handle all of that difficult scripts to get to the new url, an XML sitemap gives it all what it needs and helps you to get good results.
