Both traditional search bots and AI crawlers (GPTBot, Google-Extended) read your sitemap.xml to discover pages. If a page isn't in your sitemap and has few internal links, AI engines are less likely to find and index it for use in their answers.
Search engines treat priority as a relative hint within your own site — it doesn't affect your ranking vs other sites. Set your most important pages to 1.0 and less critical ones lower. Some engines ignore it entirely, but it doesn't hurt to include it.
The sitemap protocol limits each file to 50,000 URLs and 50MB uncompressed. For larger sites, use a sitemap index file. This tool generates a single sitemap — for enterprise sites, consider automated generation via your CMS.