Eyes are an interesting example, because they are seemingly incredibly complex systems, yet eyes have evolved independently dozens of times. For example, octopuses and humans have equivalently complex eyes which have entirely different origins and have slight differences (for example, human eyes have a blind spot due to the nerve fibers being on top of instead of behind the light receptors).
Seeing how you go from nothing to spot to pinhole lens eye is the critical part.
Basically, they go through each of those stages, and shows what it would look like from the creatures point of view
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.