Blue dot in headlight/taillight

[63 & 64 Bowties]

I know the history of blue dots has been discussed a lot before, but I don't think I've ever seen a good answer. I remember seeing a lot of late 40's to early 50's Chevys and Fords that had them (non-hot rodded cars, just mom & pop cars.) I always thought they were factory on some cars, maybe an option?
They've been around for a long time. This picture was taken in 1942....