Why choose three columns and a header?
The principal goal of any web site is to present as much information to the user before they decide to look at another site.
Many sites have a great deal of information on each page. Too much information confuses the user and makes them more likely to look elsewhere for the information they want. The three columns and a header layout enables a great deal of information to be presented to the user in a logical easy to understand form.
The information on most websites falls into the following categories. The actual content, navigation systems, graphical elements of the site such as photographs, and branding materials either for the site itself or commercial advertisements. These elements need to be separated into discrete elements in order for the user to easily differentiate between different elements.
Any branding identifying the site itself needs to be visible as soon as the user starts to interact with the page. The best place for this is therefore at the top of the page. In Western culture the top left corner is best as text runs from left to right. Extending the banner across the top of the columns sets it apart from the rest of the content.
The content of the site should be in the center of screen and other elements arranged around it. This makes it easier for the user to read and draws their attention to it. The content need not always be textual. Images can be placed here as well. As in this site the content can be sliced into several subsections to make it easy to read. This element is wider than the other elements on the site to emphasize its importance.
Navigation elements are placed on the left hand side of the screen on many sites. This in the West the eye looks at this content as it scans down the page. This makes it easy to identify and therefore use.
The remaining content on this particular site is text-based. It is mainly asides to the main information on the site. In other sites this may take the form of additional graphical elements or adverts.