On these bikes the rider sits as far to the rear as possible in order to provide the best slip streaming for the cyclists that follow.
I guess on this one as the rider sits forward, a passenger was needed for this purpose. It looks very old, so it must be one of the first bikes specifically built for this purpose.
They are called Derny bikes, here is a pic of a really extraordinary 'Derny Bike'. It was powered by a V2 Anzani engine and it enabled Leon Vanderstuyft to set a 76mph speed record at the Montlhery circuit south of Paris in 1925.