A sitemap.xml file provides search engines with a roadmap of a website's structure. It lists URLs and additional information like last modified dates, priority, and change frequency. This helps search engine crawlers navigate and index the site efficiently. A sitemap.xml file improves crawl-ability, facilitates indexing, and enhances SEO by ensuring all pages are discovered, prioritizing essential pages, and helping search engines understand the site's hierarchy. However, having a sitemap.xml file doesn't guarantee index or search ranking improvement, as search engines still evaluate content quality and relevance using their algorithms.