[HO455]

I believe it's primary reason is for the fuseable link. Which is between the battery clamp and the insulated post. If the fuseable link blows it can be replaced without cutting into the harness. I'm pretty sure 67 was the first year for fuseable links and by 73 GM had decided there was no need for an easily replaced fuseable link so it was integrated into the harness to save money and confound generations of shade tree mechanics.
Its secondary use is for one leg of the amp guage wiring. It is connected at the insulated post. Having that connected to the insulated post keeps the fuse and fuse holder away from the battery post and its corrosive fumes.