Some folks add heater-control valves because the air door(s) inside the ductwork don't seal properly. Then the A/C becomes weak, and--if the car was used in actual winter--the heater would also be weak, both from air bleeding into the desired system from the un-desired system.
The heater control valve could be there to improve A/C performance, because it's easier to cram an aftermarket heater valve in the hose, than to tear-apart the under-dash HVAC plenum.
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