i was wondering as well... it seems that a point light can cast light on every object including moving or dynamic objects(i tried). So using a point light in the middle of the sphere would work as an alternative to emission.
However, emission seem to be calculated only for static objects. In the example on the plane, not even the cubes got any red light by default. I understood that an object can only use emission textures if it is static itself also. The light probes can be used to sample light and calculate it on non-static objects.
Not sure when it is better to use emission over another source of light. perhaps having too many light sources degrades performance?