Luke Wroblewski has a must read article, Visible Narratives: Understanding Visual Organization published at Boxes and Arrows. The article shows the importance of and how to visually structure information to assist the user with finding and focussing on content they are interested in. This lesson is one that is often missed in Web site redesigns.
A visual presentation of information is an essential tool to have in your tool belt. Lack of a usable visual structure can hinder your users from finding the information they are seeking. Many users come to a new site and perform a quick scan of the information available looking for something to attract their attention as it relates to terms, visual cues, or a vocabulary that will get the user to the nuggets they desire.
The user's eye needs resting places to guide them or help the user jump from topic to topic until the user finds one topic or link draws the user (as the user believes) closer to the information. Visual organization help facilitate the user's scanning and reading.
If the visual organization uses HTML markup's header tags and CSS for presentation the information has an underlying structure. The underlying structure can be used to assist bots (non-human search tools that scrape sites looking for information) in finding information. The automated scraping or searching is augmented by the markup as the information in the headers is often given greater value and can help the information get consumed by users interested in finding and using the information. With a little bit of scripting a properly marked-up Web page can generate a table of contents. This visual structuring eases the reuse of information, which is always a benefit.