If you check inside the lens, I suspect the bulb will be right near the area where this protrusion is at. If so, the reason for this is to redirect the light so it can be seen easier from the side. It would appear from the side to be a bar of light, rather than just an overall glow. It sets it off, which makes it more noticeable. If this is the case, it is more or less a safety item put into design.
Here in the States it is mandatory for car manufacturers to have "side marker lights" which are separate from the tail lights. There are ones in the front and in the back. The ones in the front need to be amber, while the ones in the rear need to be red. These protrusions (if my hypothesis is correct) would serve as these side marker lights because they would refract and condense the light from the main bulb, making it appear as a separate light.