What is W3C Validation?
You might have come across "W3C" while checking a website's SEO performance on sites like Woorank. W3c Validation is used to check the HTML and XHTML documents and on your website for well-formed markup. Just as in SEO, web designing too has it's own updates and changes, w3c validation checks a website for old scripts and other mistakes, just as a content is proof read for mistakes.
Why Should I care about W3C validation of my website?
It makes sense to use the universally accepted codes to bring out the desired functions on your website/web app, to ensure it is easier for users and Search Engines alike. Search Engines like Google takes W3c errors as a negative factor in SEO ranking, and hence it is vital in an SEO person's interest to recommend changes to codes accordingly.