The sitemap is an XML file that is understandable for Search Engine Crawlers. This XML file is the structure of the website. The structure of the website means web pages of the website including images with alt text, graphical visual representation, and other information too.
This is necessary because of faster indexing. The submitted sitemap tells the Search Engine when the update of the web page and this will helps crawlers to crawl it and index it. One of the most important things is, using webmaster you can block your web page to not crawl and index and you have the chance to remove your indexed web page or website by using this technique.