The OEMs asked AMD for six possible outputs for DisplayPort from their notebook GPUs: up to two internally for notebook panels, up to two externally for conncetors on the side of the notebook and up to two for use via a docking station. In order to fulfill these needs AMD had to build in 6 lanes of DisplayPort outputs into its GPUs, driven by a single display engine. A single display engine could drive any two outputs, similar to how graphics cards work today.
Eventually someone looked at all of the outputs and realized that without too much effort you could drive six displays off of a single card - you just needed more display engines on the chip. AMD's DX11 GPU family does just that.