Site map is just like a road map which tells your search engine (google, bing and others) the pages you deem fit to appear when users make a search. Posts included on the sitemap are indexed and ranked on google to appear as search results.
Some websites do not use it and i do not really think it has an adverse effect but i have used it on all of my websites.
if you are a wordpress user, you can use yoast, google sitemap or jetpack to lay out your sitemap.
A sitemap is an XML file that includes your blog’s URLs. This file allows crawlers to locate all of the URLs of your blog. A crawler can follow a lot of aspects of your web, such as the size of your sitemap and the number of URLs. it is a full index that you want search engine bots to see on your blog. For WordPress blogs, a plugin called Yoast SEO, sitemaps can easily be created.
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