A lot of people believe they are demonstrating riding skill when their horse will tolerate severe equipment. When you ride with a full corridor of aids, you will never need a big bit or any gadgets to put the horse's head in a position. However, a bigger bit can be used effectively in some situations. For example, if the horse has been carried through his training with a rider who has used the full corridor of aids and the horse understands the rider's body language and positions, the bigger bit can be introduced and used for upper level games so that all the rider has to do is whisper with the reins. But even an advanced horse can be ridden effectively with a snaffle if it is ridden on a full corridor of aids.
Horses are so sensitive that they can feel a fly land on their skin.
They can feel and understand a mild bit if the rider knows how to use it. But you can't train in shouts and show in whispers. When you put a bit in the horse's mouth that multiplies your pressures you lose your corridor of aids. The bit becomes louder than your seat and legs and you lose all effectiveness. All attention is on those fingers slammed in the car door.