The purpose of standard eyeglass prescription is to give a person 20/20 vision, based on an arbitrary measurement developed by Hermann Snellen in 1862, which is much too strong. When was the last time you needed to see inch letters 20 feet away except for looking at the blackboard as a student? Actually 20/30 to 20/50 is enough for daily living and most of tasks, among which automobile drivers with licenses legally require only 20/40. Only airline pilots require 20/20.
The vision of 20/20 is that you can read the letters on the 20/20 line correctly when you stand 20 feet away from the Snellen eye chart, which is considered normal. If you have 20/30 vision, you can only see what a person with 20/20 vision can see at 30 feet when you stand 20 feet away from the eye chart. Analogically, with a 20/40 vision, you can see what a person with 20/20 can see at 40 feet when you stand 20 feet away from the eye chart.
From the above explanation, we can see that the first number is always 20 which is the distance to the eye chart while the second number is the distance at which a person with 20/20 could read the line. The lowest line that you can read on the eye chart stands for your visual acuity. The larger the second number is, the worse your vision would be. The Snellen eye chart is the standard on which most eyeglass prescriptions are based because it is considered scientific and infallible, although it is not that scientific at the beginning. In 1862, Hermann Snellen simply based his 20/20 vision measurement on the vision of an assistant whom he thought as a person with good eyesight.
Nearsighted eyeglass check are focused on and prescribed for 20 feet because all readings of the Snellen eye chart are done at 20 feet. Distance vision eyeglasses are suited for looking 20 feet and farther because eyes do not have to focus at this distance, but they are unsuited for looking closer than 20 feet because the eyes must focus to see objects closer than 20 feet.
The 20/20 eyeglasses make the nearsighted eyes adjust focus to see objects closer than 20 feet and further adjust focus to overcome the comfortlessness brought about by eyeglasses prescribed for 20 feet. The eyes are forced to adjust focus, only to result in more nearsightedness that eventually requires bifocal eyeglasses. The Snellen eye chart, upon which most eyeglass prescriptions are based, really measures one visual skill - our distance vision which is only one small part of our vision including near vision, peripheral vision, eye coordination, color perception, light sensitivity, endurance, etc. It is unnecessary to base our eyeglass prescriptions on 20/20 vision because it would eventually and finally harm our eyes.