IMHO the GPI weight of an arrow is a mute point and I have no idea why they put the GPI on shafts these days.
To give an idea what an arrow weighs. If I have a 27" shaft and the arrow weighs 9 grains per inch, I know I have a starting weight of 243 grains, raw shaft.
If I know my insert, point, fletch and nock, I don't need a grains scale.
I'd suppose for no other reason really. It's just a generic reference.