A/C Heater Box Modification?

Why is a heater hose valve needed if non ac cars didn't have them? Excuse the ignorance. Is it bad to have hot water circulating through the heater core even if the fan isn't on?

I just got in from the garage. I cut the AC side of the box off and will patch it with sheet metal. Just left the fan and the lower vent with defrost outlet. Now I just have to rig up a way to open and close the defroster/lower heater vent door.

Another question- Where does the AC control panel source it's vacuum from to send to the vacuum actuated valves?