My horse gets schooled in one here and there. Sometimes he flips his bit around and evades contact when doing hard things on the flat by opening his mouth. He used to get ridden in it a lot more, but slowly is getting used to life without the flash.
Only use it if your horse try's to void the bit. That has been my experience
I only use them on my really hot horses and that bite in group shows
Good for horses that fiddle with the bit or flip their tongue over the bit. Otherwise, they're not necessary.