It's safe to assume that you've come across the terms 20/20 vision and visual acuity. As frequently used as these terms may be, do you actually know what they mean? When you understand what they imply, you will get why your optometrist needs you to do more than simply read from an eye chart.
20/20 refers to the clarity of sight from 20 feet away. If you've been told you have 20/20 eyesight, that means that from twenty feet away you're able to accurately see that which should be seen from that distance. Alternatively, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have be 20 feet away.
Both eyes are examined one after the other. When the optometrist instructs you to read the letters on the eye chart aloud, the smallest row that you can read accurately determines the visual acuity of the eye that's being examined.
It's important to recognize that 20/20 sight actually doesn't mean that your eyesight is flawless, and that's because it only determines your distance vision. Other vital sight skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these all contribute to your overall ability to see. More importantly, someone with 20/20 vision may have unhealthy eyes. Even those who have suffered damage to the nerves inside their eyes as a result of glaucoma, diabetes, high blood pressure, or a range of other diseases might still have 20/20 vision without needing to wear eye glasses. This is why an optometrist should always perform a comprehensive eye exam, as opposed to just a regular visual acuity examination.
During your next eye exam, you'll understand why you're being told to read letters from the eye chart, and more!