What is a sitemap? Just like the name implies, a sitemap maps out your website - it is a single webpage outlining your websites structure, including sections and links. It makes navigating your site much simpler. And it is essential to have an updated sitemap for SERPs and UX.
Why does your website need a sitemap? Simply put - your website needs a sitemap in order to be found in SERPs. An XML sitemap helps search engines like Google and Bing easily understand your website's structure while being crawled. And if your website has a complicated structure, or your website is new or has dynamic pages, a sitemap will greatly improve your ranking. Not only that, you will have a list of all URLs from your website and you get crawl priority. While you are not penalized for not having an XML sitemap, it is an essential aspect of SEO.
How do sitemap errors affect your ranking? Sitemap errors include compression error, empty sitemap, HTTP error, or incorrect namespace. These errors can "dirty up" your sitemap, making your website harder to crawl. If certain pages cannot be crawled properly, then they cannot be indexed. If your pages aren't indexed, you will not get favorable ranking in Google and lose out on lots of organic traffic.