Validating your code against W3C standards accomplishes several things:

Improved Coding Skills

Validating your XHTML or HTML code ensures that you're using the properly defined methods of markup implementation. Once you start validating your code, you'll be ensured that current and future pages you produce are using the best practices available. It's also an invaluable learning tool to stop you from making the same mistakes in the future.

Cross Browser Compatibility

Using validation will strip out hacks and poor-coding techniques that may work in some browsers but display poorly in others. Validated code will help safeguard you against browsers that you haven't tested your site in and may otherwise display improperly formatted pages.

Search Engine Optimization

Validated code means optimized code; well-nested and accurate tags allow search engine robots to quickly and efficiently file pages, as well as stopping errors that might otherwise create problems with browsing robots.