It replaces the trunk and gives you a low profile trunk. They came from Germany and are pretty rare. I actually have a white one in my garage that I was debating about using. I picked it up from Lewis Preston. At the time, he said he several and made a generous deal on the sale of them. You can read a previous thread about them HERE
As far as I can remember , the standard trunk on the 1500 did not confirm to German law and they all had to be changed on the earlier 1500s until they changed the law and the original trunk box was allowed to stay , it had to do with the legal dimensions of a trunk box on a motorcycle .
Wexy is partly right. That box is a German spec pancake trunk. The windshield on early German GL1500s is also very low, the whole idea was to keep the total height of the Wing within German specs at the time.