Why are you not allowed to always keep the rear fog lights on?

You are not allowed to always keep the rear fog lights on because it would dazzle drivers behind you.

Most modern cars have one or two strong red rear fog lights. Their purpose is to make the car visible from behind when visibility is reduced due to fog, heavy rain or snowfall.

Rear fog lights are very dazzling and must therefore be turned off as soon as you are convinced drivers behind have sighted you. In other words, you must not drive with the rear fog lights on if there are vehicles close behind you.