Web Standards matter because you want your web presence available to everyone. This means making your site readable by the largest number of web browsing devices. Ultimately, every platform, every browser, every device. Even platforms browsers and devices of the future!
Web standards matter because they can significantly reduce the cost of producing web sites and also make redesigning a web site easier. It is conceivable that you can completely redesign a web site by modifying a single css file. This makes sites using web standards much easier and less expensive to maintain.
Web standards matter because pages built using them load faster and use less bandwidth. Pages with standards can be 90% smaller than pages using tables and spacers. Very quickly, that adds up.