A substantial portion of the need for glasses is caused by kids not getting enough time outdoors. Apparently some mix of very bright light exposure and long-distance focusing is required for proper eye development. (Do a google scholar search for "myopia light exposure" for the relevant studies. https://scholar.google.com/scholar?q=myopia+outdoor+children...)
I think "need" is a strong word. I have 20/70 (maybe worse) vision, but I am fully able to do most any normal task. I am even a better shot with a gun than most of my friends, with their contacts & lasic. Yes, everything is a blur, meaning I cannot recognize faces until ~30ft, but I can recognize if it is a person, a bear, or a car. Very few things are impacted by my poor vision.
I do occasionally wear contacts, so I am aware of the advantages of having better than 20/15 vision, I just usually do not require that level of clarity.
Well, we need a higher visual acuity today then just a few decades ago. To tell the difference between lowercase l and uppercase I on our tiny screens if nothing else. So small errors in eye sight play a bigger role hence more ppl with glasses.
As a side note, I suspect brains are about as good at thinking as eyes are at seeing.