Drive around my area on any dark, but crystal clear evening and you will see more cars with fog lights on than you could shake a stick at.
I drove my wife to the station this morning and although it was daylight, it was quite foggy. I didn't see one other car with it's fog lights on. Some didn't even have dipped headlights.
Leaves ya scratching yer 'ead.
I too am mystified by this. Do people put them on to deliberately dazzle other people, which doesn't work in the fog.
I have another question. Why, when the Shard is often covered in fog, was it crystal clear when I cycled up Union Street yesterday, when it was actually foggy. People approaching from other angles said it was covered in fog, but I could see right to the top.