... is that they cut down on contrast in dim light. That can make it very hard to see the edge of the road, where there is no curb, and even unpainted curbs don't show up very well. By a lucky coincidence, the clip-on lenses are shorter in height than my frames, so if I don't push them all the way down I have unfiltered vision in the lower part of my field of view, esp. to the sides. Not perfect, but for the roads around here, a necessity. Maybe the expensive "official" brands would do better, but I'm skeptical.
I'm old enough to remember when headlights were designed to help you see your way at night. Nowadays, they seem designed only to blind all oncoming traffic, almost as if they were no other reason for their existence in the first place. 
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