Why do eyes need to focus versus being able to have spread vision?

I am not positive what you are asking, so feel free to write again if I am not answering to your satisfaction. Normally, rays of light enter the eye from all angles and the brain sorts out which images it wants to focus upon sharply. There is also an internal mechanism called accommodation (focus) which helps to sharpen near images, depending upon your age and refractive error (nearsighted, farsighted, astigmatic, etc.). Simply stated, there are mental focus and optical focus factors involved constantly, and, hopefully, they are concurrently in agreement.

There are also two kinds of photoreceptors (special cells in the retina that allow you to see light and color) in the human retina. The cones are concentrated in the macula (center) of the retina and require good lighting but produce sharp images as well as good color. The rods are in the periphery (outer edges) of the retina and function at lower levels of light but do not produce images as sharp as those emanating from the cones.

Our brains use all of these mechanisms and constantly integrate them.