When a crawler visits a website, one of the first things that it looks for is the XML sitemap(s). By quickly getting an accurate overview of the amount of pages on the website, a spider can allocate a suitable amount of resources and set an acceptable crawl speed.
An XML sitemap is distinct from an HTML sitemap (which is a user-friendly link page) in that it is coded in a specific format and intended for machines.
At the very least, a website should have a page sitemap. Both images and videos should be included within the XML sitemap when a website is media-rich. Media elements can be included within a single universal sitemap, or within individual image and video sitemaps.