I am remembering back 20 years or so since I first did the 737-200 course but my recollection was that the rear outflow valve modulates to smoothly control the cabin presssure whereas the forward valve is either open or closed, as indicated by the light
On descent if the forward valve was open then the rear valve would be modulating pretty much at the closed position due to the reduced airflow at idle power which would give surges on the cabin altitude.
The forward valve therefore fully closes on descent as the throttles close and the rear valve opens to a more central position to give better control. The forward hold may get some heat from the electrics but their is no airlfow through the hold.
I think that is why on the -200 Big airways used the rear hold for freight with legs, but as I say it was a long time ago, and I think it may have been different on the 400 series and later.