Standards are very important to web browsing. When html documents (web pages) are designed to standards, then web browsers (like Firefox and Internet Explorer) are able to reliably and accurately display them. Although Internet Explorer is still a bit resistant to standards, the rest of the browsers out there adhere to them. This makes it so that your website will display correctly regardless which browser your visitors are using.
Additionally, web robots like google can more reliably index your site when it is designed to standards, making it more likely that your site will move up the rankings. Google, in particular, favors sites that use standards-compliant html. Therefore, designing your site to web standards is a simple way to get an advertising boost before you even start advertising.