For search engines sitemap - is a special .xml file that contains information about all the pages of the website. It is much easier for search engine robots/crawlers to go to all the pages if their addresses/names of them were already indicated. This is standard work for search engines like Google, Bing, Yandex, and others. All of them allow signaling the sitemap file.
The other thing is if there is a webpage that has not been linked to others and no other links which lead to it. The crawler easily never will find this page. But it is possible to indicate it through a sitemap file as well.
But there is one more description of the sitemap - the separate webpage which has "the tree structure" of the complete website to show the sections, sub-sections, and sub-pages of the website itself. It is a classic .htm / .html file like ours. It is not very helpful for the user if the menus and sub-menus of the website are clear and understandable. Anyway, it helps also search engines to understand the user's main structure if necessary.
There is no obligation to have a sitemap as a separate webpage on your website or even to signal or upload your sitemap.xml file for search engines. Robots are very powerful and can do a lot of jobs themself. Anyway, it is a good additional and helpful thing for all webmasters. Or we are too "old-fashioned":)))
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