I've had the same experience with my son recently. I feel that he has the eyes of an eagle; he can spot an airplane high in the sky, or a small insect in the air, or any little detail at home across the room -- his eyesight seems more than perfect.
But just like in your case, a check-up revealed the need to wear glasses. We got him glasses. He doesn't wear them. He seems fine all the same. But there is some science behind this that makes sense. The essence is that even good eyes can have small problems that are easily corrected early on, but harder to fix in later years.
The eyes can have defects (if I remember right, it's something with incorrect curvature of the lens) but the eyes' muscles can adjust for this, simply by applying a little more force to the lens to bend it right. This is why the eyesight seems perfect, but what we parents don't see is that the "perfect" is only achieved my muscular effort. Over time, the eyes can't continue this effort, and then the eyesight is reduced.
By correcting this situation with glasses, the eye is conditioned to see properly without muscular effort. The glasses are often as low as 0.5 or 0.25 diopter.