Html tags are mainly important while doing SEO of some website. As they inform the search engine crawlers how the HTML Site is structured. Search engine bots mainly search for the content that is in the body tag so if a body tag is mentioned it helps the search engine spiders while crawling the page.
Also the head section is important since while indexing our site in Google we need to add a code to verify the site ownership in google webmasters tool.
Also it helps the designer to code the page easily if the tags are mentioned