The evolution from photoreceptor cells to eyes served a very practical purpose hundreds of millions of years ago.
In the beginning, as is the case with flatworms today, the first eyes were merely light-sensitive photoreceptor cells.
Back then, both before, during and after the Cambrian Explosion, all animal life on Earth was entirely aquatic.
Being submerged in liquid dihydrogen monoxide is effectively functionally practically similar to being in outer space, in that water has the effect of negating the force exerted by a gravitational field such as that of the Earth's gravitational well. While these cells could sense light [https://askabiologist.asu.edu/rods-and-cones] and even tell the difference between light and darkness, they could not determine the direction wherefrom the light was coming.
The solution was to place these cells at the bottom Of a depression on the top of the head. As this depression deepened, This limited the amount of light that reached the cells to only light coming from certain directions, and thus helped the organism to determine the direction the light was coming from.
Many aquatic species such as krill are diurnal, meaning that they spend the daytime in the deep water where it is safe from predators, but rise at night into the shallow water to feed. The ability to tell day from night is crucial in this life cycle [https://www.sciencedaily.com/releases/2013/02/130206190630.htm]. Krill that did not descend to deeper water during the day were vulnerable to predation. Krill that did not rise to shallow water at night starved.
While photoreceptor cells were capable of sensing light and distinguishing light from darkness, a crucial trait for the diurnal lifestyle of many aquatic animals in their ability to distinguish night from day, it was only when these photoreceptor cells were placed at the bottom of a depression [or "eye socket"] with a relatively narrow pinhole opening [or aperture] that these animals gained the capability to determine from which direction light was coming. This is important in the near-weightless environment underwater because, like in outer space, without gravity, there for all practical intents and purposes exists no "up" or "down".
Needless to say, hundreds of millions of years before the invention of electricity, the only source of illumination on the Earth's surface was sunlight. As such, at least during the day when the sun was directly overhead shining down on the water, for the animals in the water, the direction the sunlight was coming from was "up". This became especially important later, first with the evolution of aquatic reptiles such as ichthyosaurs and later with the evolution of aquatic mammals such as cetaceans [dolphins and whales] due to their need to breathe air and their tendency to give birth to their live young underwater. But even for primitive aquatic animals such as the aforementioned krill, the ability to determine the direction of the sun [or moon] was a vitally important survival trait.
From an eye capable of determining the direction of light [requiring a depression with photoreceptor cells at the bottom] it really is a relatively short evolutionary leap to an eye capable of forming images.
The narrowing of the opening in the depression eventually formed a pinhole, which was ultimately covered by a lens. With the now-enclosed eye socket filled with water, this allowed the photoreceptors to not only sense light, but form images.
The principle of using a narrow pinhole aperture to project images of the outside onto a screen in an otherwise dark room has been understood since the Italian Renaissance of the fifteenth century. They called it the "camera obscura".
Since organisms with clearer vision had a distinct advantage, through successive generations, multiple adaptations made these images progressively sharper and more detailed.
Of course, since hundreds of millions of years ago all animals were aquatic, the animal eye evolved to function underwater. As any swimmer knows, just as water vapor in clouds refracts light to create a rainbow, being underwater distorts images of things above the surface [or, seen from above, distorts objects below]. And yet, a little over a third of a billion years ago, animals such as Tiktaalik [called "amphibians"] left the water.
The problem faced with adapting eyes evolved to function underwater to use in air was much the same as the problem faced in adapting the amphibian practice of laying their eggs in water to allow them to move further inland. The solution, too, was strikingly similar: take the water with them In the case of reproduction, this led to the evolution of animals with scaled skin that retained moisture who laid eggs containing seawater-like amniotic fluid contained within a hard outer shell [these animals are what we call "reptiles"]. In the case of the eye, it resulted in the evolution of a spherical organ composed mostly of water [resulting in its tendency to get dried out and need near-constant moisturizing] contained within a membrane. This organ is what we call the "eyeball".
The automatic involuntary subconscious process of cleaning and moisturizing it is what we know as "blinking". This is why fish do not blink, or even close their eyes when they sleep.
In mammals, there evolved an additional mechanism for moisturizing the eye. We call them "tear ducts". Mammals also evolved an additional defense mechanism protecting the eye from dirt and dust: eyelashes.
At each stage of this process, a more advanced eye created a definitive survival advantage for the organism with it over those without. An organism with eyes was better at hunting its prey and avoiding its predators then an organism without eyes.
1.) The evolution of the eye, from Scientific American: http://www.scientificamerican.com/article/evolution-of-the-eye/#
2.) The evolution of the eye, from the University of California--Berkeley: http://evolution.berkeley.edu/evolibrary/article/eyes_01
3.) The evolution of the eye, from the National Geographic Society: http://ngm.nationalgeographic.com/2016/02/evolution-of-eyes-text
4.) The evolution of the eye, from the New York Academies of Sciences: http://www.nyas.org/publications/detail.aspx?cid=93b487b2-153a-4630-9fb2-5679a061fff7
5.) The evolution of the eye, from the University of Utah: http://learn.genetics.utah.edu/content/selection/eye/
