According to Pontiac Enthusiast, Vol 1 No. 2, 1968-69 Emissions Systems Tech Part 2 TVS, by Peter Serio (possibly he will also respond here) the TVS has two purposes. First, it is a safety device to help prevent overheating. "Whenever the coolant temperature rises above 230 degrees F, the TVS switches the distributor advance from ported to full manifold vacuum. This advances the timing about 20 degrees at idle, allowing the engine to cool down to normal operating temperature."
Second, in some '68 models there was a vacuum retard to reduce emissions