The highway code states that fog lights are to be used when visibility is seriously reduced, generally when visibility is less than 100m. Not solely when it's foggy.
When it's snowing heavily, in case you haven't noticed Mike, visibility tends to be a few feet in front of your car with normal headlights; last time I did maths a few feet was quite a bit less than 100m. Foglights are much lower down and illuminate more of the road, with less of the glare from the snow. Having the rear one on gives the car behind chance to see your position further up the road, whilst he's straining his eyes to see what's on the road a few feet in front of him.
This is common sense, steve32c has it.