That pipe is the ambient pressure source for the N75 and N18 valves, which control the positioning of the VNT vanes in the turbo and the EGR valve in the inlet.
They switch between vacuum and ambient, low and high pressure, to create a specific intermediate pressure which makes the vanes or EGR valve open to a specific, intermediate, position. Without a source of ambient, normal-pressure air for reference, the turbo vane and EGR valve positions will go haywire.
Get the screw out and find a suitable adapter to plug the pipe into the inlet just behind the filter,which will be the nearest equivalent to the position it was in in the airbox.