Yes the one on the radiator. It helps air flow across the radiator and condenser which aids in reducing the delta between the high side and low side of the AC system much like the exterior fan on a heat pump home heating system. Without that airflow the high pressure side will get too hot and then the low side will not be cool enough to tell a difference. You will reach the high side over pressure switch and it will disengage the compressor. I am not the best at explaining refrigeration theory.
Our Crown Victoria when placed on Max AC will always turn on the electric fan attached to the radiator. When on normal AC it will rely on the thermal clutch fan and normal air flow of course until the radiator temp gets hot enough. All manufacturers have a strategy for how they operate the AC for best performance.
We had a 01 Galant, AC cooled fine sitting still in park or neutral. Put it in drive and sit at a stop light the AC would blow hot. That was a battery that would not pass a load test. The radiator fan would slow down while in gear because Mitsubishi didn’t charge the battery when in drive and idle, at least that car did not. When the battery was weak not enough airflow.
Other issues could be the amount of bug/dirt buildup on the condenser or maybe even damage to fins on the condenser but that would be a cooling problem all the time I believe.