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 broken links.

If you need more than the this tool provides then keep a look out for the full version that allows you to test more than a page at a time, has more options for each validation and can be optionally run from the command line.