you could try and argue that an electric fan on the radiator is no bigger than that desk fan. but they are placed on the radiator to directly suck or blow through it. something like that desk fan will just blow around it.

and usually when your engine is producing max power and needs the cooling the most (or just after) it is travelling at speed so has the wind going through the radiator. during fast driving, an electric radiator fan should not come on whilst on the move, only when you come to stop.