Its easier for search engines to crawl around and neatly inspect your content, giving you some brownie points . But its not that very important , even facebook is not w3 validated. W3 validation is however important wit regard to mobile web but that is a different story all-together.
if your error will solve in W3c so you can get good result in search engine bcz all search engine want''s W3c clear website
There is no real significant benefit of ensuring that your site is W3 compliant in terms of SEO, it's just good practice.