For most of the '90s and the first part of this decade, content providers who wanted to publish online only needed to worry about the graphical web browser.
Because the competitive landscape of the web is such that the site which looks and works best gets the most traffic, developers and designers put a premium on the presentation of that content and let structural markup take a back seat.
Our old site did not have very good support for the disabled, but our new site should soon have much better support. With all of our content in divs now, we can hide all but the relevant chunks of content and navigation with a simple alternate CSS file.
Now that digital lifestyle devices, tablets, wireless phones, and other Internet appliances are beginning to come of age, we need to worry about presenting our content to these devices so that it is optimized for their display capabilities.