The eye is a lens that focuses rays of light from a limited spectrum, after which the the image is turned over to the brain receptors that interpret it (beginning with turning it the right side up first, I think).
It is a complex and fragile instrument whose sole purpose is to pre-process the image "mechanically" (rather than "electronically") before turning it over to the "CPU." Please bear with me.
Most creatures who have one eye usually have another one as well. This second equally complex piece of hardware has, it would seem, two purposes: one, it serves as a spare; two, it allows its owner to see depth a bit better. (You can still see depth if you cover up one eye: your brain has to make a few adjustments, and a few more assumptions, but it's not like depth perception is completely gone, right?)
Now think of the original camera obscura (the one that had no lens, just a hole). By adjusting the size of the hole (aperture) one could make the resulting upside-down image on the opposite wall a bit sharper. The image is still a bit pale, but the subtle differences in color values would still be enough for a powerful computer (such as our brain) to work out the nuances.
Why would then God (or Evolution, for those who still believe in it) opt for making an infinitely complex piece of hardware (the eye) and then furnishing a backup as well, instead of just adding a few lines of code to the operating system?
Please don't take any of this too seriously.