With the long bowl, it gives the diodes ‘room’ to light the road below, but at the same time, without a lamp in there it’s just pointless wasted space!! 

They look much nicer in daylight though. I always prefer the look of a post top which actually has the lamp hanging down in the bowl though, the modern HID ones with a reflector up inside the canopy with the lamp horizontal look a bit odd with an empty bowl.