I think the addition of the diffuser adds the 'depth response' to the camera. Imagine that you had two point light sources located some distance away from the s

I think the addition of the diffuser adds the 'depth response' to the camera. Imagine that you had two point light sources located some distance away from the sensor such that one is directly in front of the other. They're magical point light sources so the light from the rear one just passes through the front. If you didn't have anything in front of the sensor, there would be no way to distinguish that there are two point sources, as opposed to a single point source with a non uniform output.