Interesting how a simple question can go in so many directions.
I'm told the point of the flash, and the dropped noseband, is to help keep the bit correctly placed in the horse's mouth (certainly not to keep the mouth closed.) In real life: it's used to keep the mouth from gaping, as well as putting pressure on the face when the horse pulls or tries to charge ahead. In other words, a restraint, not a a piece of tack like a girth.
Beginners should be on safe horses. There's so much to learn about how to handle their own bodies, it's too much to expect them to be schooling a horse as well. Real life: you often come across a rider whose horse is too much for her.
The first time I ever heard that a noseband can keep a horse from breaking his jaw was from a professional reiner. I was quite surprised; most of the local Western riders don't use any noseband at all, but at the clinic, they all had to use nosebands.