I wonder if the mouth protector was something they actually wore in battle? Looks like something used post death.
It was a bite-down to give some protection to the mouth (a good place for a spear to go, once in, it goes all the way in without much guidance; there’s a description of such a death in the Iliad) and throat. The gold may indeed be for grave goods only, but gold has to taste better than bronze any day of the week, and after the sack of Persepolis, Alexander became the wealthiest man in the world.
There’s a recent FR topic regarding the use of linen armor by Alexander’s troops, which would have been much cooler to wear than leather, and may have worked better as well.
Especially gold. Gold won't protect from much of anything. It's too soft.