Many a year ago, Jeff Rouyer won the Netscape DHTML competition with his flying dude interface and put DHTML on the map. The winning entry, apart from being 3D in design which up until that point had not been seen on the Web, also had something that was quite uncharacteristic of Web pages. Rather than have a user traverse across multiple Web pages which consumes resources by opening separate window instances, Jeff was loading content dynamically into an existing interface. In other words all the content was being loaded into an existing element and all other elements were not being changed or swapped. We are talking pre IE4 days here.
IE4 was released a little later to challenge Netscape's dominance on the web, but it did not have a similar method where content could be loaded directly into a layer element.