From Total Validator
Why use separate validation tools to check your web pages when you only need one?
The Total Validator Tool performs the most important validations on your web pages: True (X)HTML validation against published W3C/ISO DTDs; accessibility validation against the W3C and US Section 508 guidelines; spell checking against one of five languages; and testing for broken links.
Why these 4 validations
Search engine robots/spiders are very fussy and will not search or index some or all of a site if they come across invalid HTML. So if you want to be properly listed you need to be sure your HTML is valid.
Just because you can use your website doesn't mean that someone with a disability can. You should ensure your sites conform to the W3C WAI Accessibility Guidelines, (and that also means strictly W3C validated HTML), or US Section 508 standard, if you want your site to be accessible to all.
Spelling is often left unchecked and will make your site look and feel unprofessional and even unusable.
Broken links are the bane of every webmaster, you may get your internal links correct, but external sites tend to change all the time, and there's nothing that makes your site look worse than a link to nowhere. So you need to regularly check your site for
# Added support for IE9 screenshots
# Added HTML4+RDFa 1.1 validation
# HTML5 updated to October 19th version
# (X)HTML5+RDFa 1.1 updated to Oct 19th version
# XHTML+RDFa 1.1 updated to Nov 9th version
# XHTML 1.1 SE updated to 7th October version
# Basic 1.1 SE, Print SE updated to 7th October version
# BUG: No longer checks links that use protocols that cannot be followed
Java 1.5.0 or later