There is so much more to a modern engine than just 3D graphics. Sound, input, windowing integration, high-speed disk streaming IO, networking, robust OS integration (where to store save files, etc), just compiling it. Problem is that testing most of this depends on having a working renderer to display the game. This tool allows getting something on the screen faster so you can test everything all at once instead of having a bunch of sound, networking and platform engineers waiting around for the rendering team to get their part working.