It appears its much the same as indexing your spark plugs.
Marking the nut flats that coincide with the window on the on the needle and seat so it points towards the fuel feed side of the bowl.
See post # 4 for instructions.
BLP bottom feed needle and seats - Chevelle Tech
Indexing Needle and seats - Ask the Carb Guy
I imagine it could be beneficial if you road race you car (sustained high fuel consumption). Probably not so much Kmart parking lot.