U.S. Air Force required the test when I applied a long time ago. Allowed a better inspection of the eye.
Several doctors have stated that looking into the eye is like looking inside the body. Several diseases can be seen in the eye before it becomes noticeable(cardio vascular disease being one?)
FAA doctor found a spot on my eye several years ago. Possible cancerous so he wanted it checked out. Required eye dilation by eye doctor for better exam. Had to do that once again several years later for routine followup. No big deal. I drive myself home afterwards(3 times)
You look crazy afterwards with your eyes completely dilated. It looks like you have completely black eyes. Folks ask "are you OK?"