CSS supports aural style sheets, which specify how a document will sound when rendered as speech. Aural style sheets (or "ACSS" for short) allow authors and users to specify the volume of spoken content, background sounds, spatial properties for sound, and a host of other properties that can add effects to synthesized speech analogous to those achieved with styled fonts for visual output.
However, implementation of ACSS support in user agents is still sparse.
Unstructured pages make content indexing inordinately difficult. A truly powerful search engine would allow users to search just page titles, or only section headings within pages, or only paragraph text, or perhaps only those paragraphs that are marked as being important. In order to do this, however, the page contents must be contained within some sort of structural markup -- exactly the sort of markup most pages lack.
A lack of structure reduces accessibility. Imagine that you are blind, and rely on a speech-synthesis browser to browse the Web. Which would you prefer: a structured page that lets your browser read only section headings so you can choose which section you'd like to hear more about; or a page so lacking in structure that your browser is forced to read the entire thing with no indication of what's a heading, what's a paragraph, and what's important?
Advanced page presentation is only possible with some sort of document structure. Imagine a page in which only the section headings are shown, with an arrow next to each. The user can decide which section heading applies to him and click on it, thus revealing the text of that section.
Structured markup is easier to maintain. How many times have you spent long minutes hunting through someone else's HTML (or even your own) in search of the one little error that is messing up your page in one browser or another? How much time have you spent writing nested tables and FONT tags, just to get a sidebar with white hyperlinks in it? How many line-break tags have you inserted trying to get exactly the right separation between a title and the following text? By using structural markup, you can clean up your code and make it easier to find what you're looking for.