W3 is a free service that check the validity of web documents for conformance to W3C standards. Validating document is an important step to help improve and ensuring the quality and can save time and effort. W3 is a trusted link and can boost traffic for the site.
Well first off, I don't link to w3. Second off, by validating your sites' coding, it ensures proper indexing in search engines. Just another SEO technique..
I do on this page http://www.apopularitycontest.com/advertise.php just so prospect customers may trust more.
Well, they will not trust you much if they click the link I dont link to them as only webmasters know what the link is when they see it.
It's simply a sweet looking little button for spending the extra time to get approved. Ever seen how many of those little buttons godaddy has? Bob Parsons is a big fan of them.