There isnt much to explain really. Stone voice refreshes the freeze duration on a previously frozen giant. If youre able to freeze a new giant and an old giant at the same time (they need to be fairly close to each other), and then repeat until you have four giants, you prevent new giants from spawning.

No more kiting. No more accidental giant kills. Person with shriek just hides behind the pile of giants.

Different groups might feel differently about this but my group feels it isnt worth the hassle.