I believe it's to recycle any oil that separates from the airstream that will settle in the box section of the intake.
As an engine cools, any oil in the air inside the intake will condense on the walls and collect in the bottom of the box. This directs it back into the airstream during engine operation, cleaning out the liquid and vaporizing it again.
With the large area of the box section, there may not be enough flow during engine operation to pickup the liquid and vaporize it. Draining it to the main inlet tube where there is increased flow allows it to be vaporized again for burning.