Validating a website is the process of ensuring that the pages on the website conform to the norms or standards defined by various organizations. Validation is important, and will ensure that your web pages are interpreted in the same way (the way you want it) by various machines, such as search engines, as well as users and visitors to your webpage.
Conforming to standards and regulations is one of the many ways you can make your website universally understood. Make sure your codes and styles validate across the board. That means they have to meet the “strict” standards set by the W3C Organizationand pass a variety of validations for CSS and XHTML.
Not all validators check for the same things. Some only check CSS, others XHTML, and others for accessibility. If you are sincere in presenting standardized pages to the public, test them with several validators. The World Wide Web Consortium sets the standards and also hosts a variety of web page validators.
In simple terms, validation ensures that your website complies with the standards accepted by most web designers. That also means that it will be accessible to more people, across more web browsers and operation systems. Having an accessible website is also regarded as good web design practice. As you can see, having a validated website has its definite plus points.