Looking at it I'd guess it is to replace the pillion seat and give the bike a sportier, 'Ducati' style look to it. The smaller piece looks like it's to go over the top of the tail light!
It fits on the rear platform when the rear seat and the rear rack is removed a bit like the seat hump on a sports bike. I had one that came with a set of panels I bought for mine but never bothered fitting it as I use the rack too much.