it is rather important as website structure and coding is still considered as a ranking factor. 100% correct code can not boost up your rankings, but with bad code you may rank lower than you could. Check here for more details.
As I know the seo I think every factor has its own importance to do and so I think validation is a smart thing to do. You want to make sure your site works before the distribution and import of their work.
W3C validation is indirectly related to SEO and does not affect SERP’s ranking directly. Since, W3C validation means free from code all code errors, It increases - - the visibility of a website on other browsers - credibility of your website
The website becomes compatible with all browsers, thus one can target - vast audience - physically impaired people - people who use portable devices for surfing
Validation is important to get your site free from errors. W3C validation is a tool to check the errors present in the site, so that the site can be error free and ready for SEO activity to be performed.
