I believe the logic is that if you say "good!" is the hen taking what she was doing when you said the "g" or what she was doing when you said the "d", a click is much more sudden, it is also unlikely that you will click randomly all of the time when around the birds and confuse them, if you use your voice they MAY think everything you say is directed at them.You always have your voice with you. Do you always have your clicker? Clickers were originally used to train dolphins, which happen to click to communicate. Yes you can use a clicker to train other critters, but why? You can also use a whistle or a trumpet, but why? Dolphins hear a clicker better than a voice. Your chickens can hear your voice. Save a step in training and use your voice. My opinion.
I can see both sides, but that's one reason for a clicker over just voice.