Because hens know his language and they beat the stuffing out of rude males in a way that he can understand. Humans have different languages and behaviors than chickens and they generally can't beat the stuffing out of a bird without consequences
Plus some chicks are just more fragile and can die under a broody's care when they may have survived in a controlled brooder. Not to mention predators can snag them, other birds in the flock may not want them around, the weather can change if they can't fit under mom....
I don't think there is a 'safest' way. I lose incubator chicks and I lose broody chicks. But I also have incubator chicks survive and I have broody chicks survive