Web standards have been developed by the World Wide Web Consortium (W3C) to aid functionality, accessibility and performance of your site. Benefits include...
Improved search engine performance (eg. Google). A complient web site helps google index your site resulting in higher page rankings
Improved in site search facility (if applicable) helping your visitors find your content quicker and more efficiently.
Improves an older browser´s ability to display the content of your web site.
It´s easier for people with disabilities to view your web site. For example, blind people can use computers that read web pages for them and partially sighted people can enlarge the text on your website.
Helps intergrate a Content Management System (CMS) or e-commerce (Shopping Cart) into your site.
Improves your site´s ability to be viewed on devises other than a PC, such as a mobile phone, PDA or TV.