Martin,
It serves two purposes, first, its to contain the light emitting through the courtesy lens, otherwise if the trunk were open as well as a door the light would be seen above the rear wheel wells. That lighted area is shaded not to draw the owner to think it's useable space. Secondly, the tube keeps moisture off it from the drain tray above it from the side vent. The drain tube runs next to the tube.
Cheers,
~Earl J