In 1, light from any direction will fall on all the pigment cells equally. Essentially, your camera only has a single pixel--you can't tell direction at all, only degree of brightness.

In 2, light centered on the cup will fall on all the cells, but light from the side will be occluded by the edge of the cup and only fall on one side. Now you have a low-resolution camera, and can roughly tell which direction light is coming from.

In 3, light from any angle will only light up a small part of the cup's interior, allowing for greater precision--effectively, higher resolution.

From 1 to 2 you get directionality. 2 to 3 improves directionality a great deal (it's basically a continuum from depression to pinhole eye).

More light collection and higher resolution of details