The A/C controls tell the ECU when the A/C is turned on, this way, the ECU can:

1) Be prepared to throttle up for the additional load.
2) Command the fan to run so the condenser has air flow across it.
3) If the ECU senses any issue with the engine fan, it tells the A/C controls to not engage the A/C, as the additional heat and workload of an operating A/C are not a good idea with a compromised cooling system.