An XML sitemap doesn't necessary affect your search engine rankings, unless your website is not being indexed well. That's what a sitemap does: it helps search engines to index your site, making the indexing process faster.
Is it a sitemap necessary?
Simply put, a sitemap is a file that lists all the URLs of your website, created in XML(Extensible Markup Language). It’s a Search Engine Optimization tool for indexing hard to reach pages and new content.
The sitemap will help you when: