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...