Use it or lose it

   Take it this way: if you don't use your eyes, then they can't develop, if talking about early years, or they will deteriorate.
   It is the same as with any other part and organ in our body. Look for example in muscles training. If you want to have strong and pumped up muscles then you don't sleep or watch the TV all day long, but you go in gym to exercise the muscles, those you want to develop. If you want to make stronger arm muscles you would lift weights. Meaning you regularly use those muscles, activate them, you repeat these operations, and as the time goes - what happens - they became stronger, nicely formed, etc...
   Another example is learning. We all know that we have to repeat newly learned knowledge if we want to remember it, otherwise we forget. So, as long a we go back and repeat some learned knowledge we will know if. For example you learned in maths how to calculate area of the 2D shape, if you don't repeat this some required number of times, you will most probably not know it until you go and learn the theory again (or someone tells you - that is the same as learning the first time - this time can be just quicker).
   The same I would say with eyes, if you don't try how do I see close, distance, left-right, up-down, etc then you just can't see properly, the object seen will be blurred.
   With trying the brain makes connections. As I see the eyes function it is constant feedback cooperation between eye as organ and brain. If brain didn't see the object properly it will send another command to the eye to change shape - whether for closer or further viewing.. etc. until brain is OK with what it has seen.