Why test at all if your website looks okay in your browser? Here are our top five reasons:
Number One: Accessibility. Just because you can use your website doesn't mean that someone with a disability can. For many countries this is not only a moral, but also a legal requirement. So you should ensure your site conforms to the W3C Web Accessibility Guidelines, or US Section 508 standard, if you want your site to be accessible to all. Conforming to these standards also means strictly validated HTML.
Number Two: Search engines. These can be very fussy and will not search or index some or all of a site if they come across invalid web pages. So if you want people to find your site you need to be sure your pages are validated.
Number Three: Happy visitors. Who wants to come to a site that looks bad in their browser, has broken links, and is full of spelling mistakes. IE's dominance is fading and each browser has its own quirks, but all are striving to conform to the standards. So make your site standards based and it will work better in all browsers and for more people. Check for broken links regularly and for spelling mistakes too. Annoy or upset your visitors and they are less likely to return.
Number Four: Maintenance. You've probably heard the term "Internet Time". On the Internet things move fast, and a validated site will be easier to maintain, easier to improve and so easier to keep up with the latest trends.
Number Five: Tools and CMS. Authoring tools may make it easy to write and maintain pages, and content management systems that generate pages on the fly seem cool. But almost all will produce invalid pages without you knowing and so you may already be failing to meet many of the reasons above.