From the Kemper website:
We spent considerable time researching the dynamic behavior of the tube. It was also necessary to study the interactions between the tube and surrounding circuits, which become very complex when the tube starts to distort. We were lucky to find a way to extract this exact tube behavior by analyzing the intermodulation products of crossing sine sweeps. The relation between the intermodulations and the unprocessed sine sweeps tell us the full story about the distortion shape and dynamic of the tube and the behavior of the surrounding circuit. It can even deliver the information about several distortion stages in a row, so long as there is only one stage significantly distorting. This measurement is independent of the frequency response of the amplifier. Once the distorting part has been analyzed, it can be separated from the transfer function. The frequency response of the cabinet is then easy to determine and separate.
Aside from the ease of profiling the whole amp, there is another good reason not to analyze the amp part by part: we capture the sound of every component at the place where it belongs, including all interactions between the components.