Its for the high intensity rear fog light. This light is mandated by law in England and Europe (since 1980 I believe). A vehicle must have at least one - some have two - of these lights to use in fog/inclement weather. In countries that drive on the left (England) the light (if it has only one) must be on the right, and in countries that drive on the right the light must be on the left. On my GLHS it is fitted neatly into the light cluster taking out one back up lamp. On the SC it is hanging down on the right hand side below the bumper. Personally I very rarely use these lights. Most people put them on and forget....and leave them on to dazzle!!
