So how important is a sitemap? Does the inclusion help in SERP/SEO? Is the exclusion derogatory? Just how important is it to have?
site maps help the search engines find all the pages on your website, so I would say it's pretty important
have a sitemap on your website is very important, robots can crawl faster and index all the pages of your site.
Sitemap is to index all the page in the site not for getting traffic or ranking. If don't do site map also site will indexed but will take little more time