The "when" of the rear fog light generally is: Only in times of very low visibility/dense fog. The light is intended to provide an extra measure of visual range, that is greater than the tail lights themselves, for rear approaching traffic.
The word "The", above, is emphasized because only the rear fog light towards the center of the road is to be on. The left light in most of the world, the one on the right in Blighty and other countries afflicted with driving on the left.