if you remember, there had been a thermostat *** on the windows ACs whihc were used earlier in the houses before the emergemnce of splits ACs. This knob either had the numbers from 1 to 5 or cooling levels of Low, medium and HIgh; Purpose being to control the cooling temperatue level in the room.
Same thermostat switch is also placed in the cars to have thed esired cooling level in the pasenger cabin when AC temperature will reach the desired level it ill trrip the AC compressor.
In absence of it, compressor will be working contiuously putting continuous load on the egine, hence reducing its performance life.