What’s inside
In addition to the page URL itself, the file may contain optional fields lastmod, changefreq and priority. Google and Bing explicitly state that they completely ignore priority and changefreq – the only field that is actually used is lastmod, and only if the dates in it are reliable rather than automatically set to today. On website builders, sitemap.xml is usually generated automatically and updated when new pages are added.
What this means in practice
Auto-generation does not eliminate the need to check the file manually: technical or duplicate pages that should not be indexed may be included, or pages added in a non-standard way may be omitted. The file should be regularly compared with the list of URLs actually needed for indexing.
Who it is critical for
Important for any website, but especially for large ones, where the crawl budget is limited and the search engine needs to be explicitly told which pages to prioritize for crawling.