Why validate your website if it 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. Making sure your site is accessible is not only a moral requirement, but in many countries a legal requirement. So you should ensure that your site conforms to the W3C Web Accessibility Guidelines, or US Section 508 standard if you want your site to be accessible to all. Note that conforming to these standards means strictly validated HTML, which dedicated accessibility checkers tools fail to check.
Number Two: Search engines. These can be very fussy and may not index invalid web pages or those with with broken links. So if you want people to find your site, you need to validate it and fix any broken links.
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. Each browser has its own quirks, but all are striving to conform to the standards. So make sure your site follows the standards, so it looks its best. Regularly check for broken links and for spelling mistakes. Annoy or upset your visitors and they are less likely to return.
Number Four: Maintenance. Constantly updating and improving your site will keep people returning to see what's new. A validated site will be easier to maintain, easier to improve, and easier to keep up with the latest trends.
Number Five: Tools and CMS. Authoring tools make it easy to write and maintain pages, and content management systems that generate pages may speed up development. But by default almost all will produce non-standard and inaccessible HTML. So you need to regularly check their output, if you wish to meet any of the reasons above.