'Why does website validation matter?'
Google's Webmaster Team is responsible for most of Google's informational websites. They state "Why does validation matter? There are different perspectives on validation - at Google there are different approaches and priorities too - but the Webmaster Team considers validation a baseline quality attribute. It doesn't guarantee accessibility, performance, or maintainability, but it reduces the number of possible issues that could arise and in many cases indicates appropriate use of technology."
'But my website looks fine!'
A website doesn't look like anything at all! It only takes on a visual appearance when it is presented by your browser. Many systems (such as Google's search) don't see your site design at all - they just see the raw code, and potentially code littered with errors.
'Many websites don't validate!'
Do you want to risk being on the wrong side of a lawsuit if your site proves inaccessible to a disabled person who cannot use a 'conventional' browser? Accessibility is the law in many countries.
'How can I check the validity of my website?'
Visit the W3C website validation tool, or contact us and we'll let you know whether it's invalid and any issues that need resolving.