Peter 2 replied:

I used to put one on the front of the rack on the troopy, made a big difference to the wind noise and stability and in so doing it probably helped fuel economy.

Alan H (Narangba QLD) replied:

I found it also used to help weather proof the load by preventing wind blown rain as such and helped some vegetation clearance of the rack. If you drive in heavy vegetation it will be a pain if it gets between the roof and the rack.

Lately I don't know - can't remember driving in rain it has been so long!