Descent with modification requires a sequence of steps by which a structure could evolve from some earlier structure. The complexity of the eye is possible via descent with modification from structures like light-sensitive spots, which are evolutionarily useful even if they do not make images for "seeing" as we often think of it. You can easily imagine a series of steps allowing more sensitive or precise light sensation, followed by the appearance of some kind of lens/focusing-mechanism, could also be selected for once they appeared in a population.
Work Step by Step
For an example of a mere light-sensitive patch being adaptive, see the limpets discussed in the text. They are molluscs, a group with members (cephalopods) that have much more sophisticated eyes. You can also see the primitive eye spots of flatworms and the more sophisticated insect eyes found in the same large group of animals (protostomes).