W3C validation is an important factor to take into consideration when you are on the lookout for errors that break your site's functionality (or ones that might potentially break something). It's something of a radar that might detect errors before they start breaking your work.
For instance, if you have a problem with your site and want to post a question on SO about it, you're well advised to use the W3C validator and make sure it doesn't point you to the source of your problem.
But... not all HTML will validate. The most important, though not the only case is probably HTML5. In CSS, vendor prefixes will not validate either. Does that mean you shouldn't use them? Not at all!
Validation services are a great thing to find that mistyped attribute name, unclosed tag, missing semicolon or a
