The 5-port switch was used on the 1969 auto trans Pontiacs. Its intent was to divert (or bypass) vacuum to the vacuum advance when the engine temp rose to 230 degrees. The bypass would allow another 20 degrees of ignition timing to cool the engine down.
The same function was handled by the 1968 dual-port vacuum advance cans.
It's not a necessary part, but of course it's a desirable part from a restorer's point of view, because it's right up in front of the intake manifold in a highly visible location. Currently, 3-port units are available, but the 5-ports are not (unless it's changed in recent months). I sold two of these 5-port units, both of them were aftermarket parts I picked up in the 1990's, for $125 each on E-Bay. Factory GM units fetch $150-$200 or more.
If you run full vacuum advance, there's no need for the TVS (thermostatic vacuum switch).
BTW, in later years, the TVS unit not only controlled vacuum advance, but also when to engage the EGR solenoid.
