Let's start with "why." Why was the Herbst appliance invented? It corrects overbites, a condition that has been traditionally treated with headgear because braces alone cannot remove an overbite.
The Herbst appliance is meant to replace headgear. But why? Well, headgear works but has some obvious problems.
It's only effective if worn for long hours (12 to 16 hrs a day).
And it's fairly distracting visually.
The Herbst appliance performs the same function as headgear, gently removing overbites by guiding the growth of the jaw. However, the Herbst appliance is completely contained inside the mouth, solving both of the main drawbacks associated with headgear. Since it works in conjunction with a growing jaw, the Herbst appliance cannot be used on adults.