That is Image Based Lighting (IBL) where the surrounding mesh is used to calculate the quality and intensity of light based on the image attached to it. Using a cube to enclose the scene is almost the opposite. It does not, of course, a source of light; it limits the calculations required in tracing the light from the mesh(es) that are the source of light inside it. You may also have parts of your scene model outside the cube, those that will not be seen, and calculations will not be wasted on those parts. Yes, the IBL sphere itself would be too large for that purpose.