Last Lemming,
Because, as Brf said, the CI allows very minute changes to both the VTF and azimuth. I suppose the ring is theoretically supposed to enable small adjustments, but since it's much heavier than the CI, it is not as easy in practice. Also, I personally did not like using the ring for setting the azimuth. You have to loosen the screws, and I found that twisting it back and forth to achieve minute changes would cause the ring to slide down after a while and affect tonearm balance.
Look, VPI arms can certainly be adjusted with incredible precision without the CI, the longer you've had one the better you're going to get at adjusting it, most likely. But the general consensus is that VPI arms are rather finicky to adjust. The CI simply allows fast, headache free, minute adjustments, which can make or break the sound. I don't claim the thing is absolutely necessary and essential if you own a VPI table, but its utility cannot be disputed.
