[smallbeer]

Quality Assurance - although the site may render fine today even though it doesn't validate, unless you validate against the spec, there's no way of knowing that it will render fine tomorrow.

Accessibility (to a point) - web browsers do a good job of rendering pages even though there are errors in the code. Some other devices may not.

http://www.w3.org/MarkUp/2004/xhtml-faq#whycare

Why should I care if my document is in correct HTML? It displays all right on my browser.

All browsers know how to deal with correct HTML. However, if it is incorrect, the browser has to repair the document, and since not all browsers repair documents in the same way, this introduces differences, so that your document may look and work differently on different browsers. Since there are hundreds of different browsers, and more coming all the time (not only on PCs, but also on PDAs, mobile phones, televisions, printers, even refrigerators), it is impossible to test your document on every browser. If you use incorrect HTML and your document doesn't work on a particular browser, it is your fault; if you use correct HTML and it doesn't work, it is a bug in the browser.