Funny enough, I posted my headcanon on how Mokou got her hair amulets in the headcanons thread a few days ago. :V My thought was that Kaguya gave them to her on a whim, but, thinking more seriously into it now, doesn't she have the Robe of the Fire Rat?
One possibility is that Mokou's trousers are made from the Robe, and the amulets are cosmetic---heck, they could even be patches covering holes---which she replaces when damaged by danmaku. Doesn't Tojiko, whose clan has history with the Fujiwara, have them too?
As for why I entertained the possibility that the amulets don't do anything, I was under the impression that the fire-warding attribute is a widely accepted explanation, but not the canonical one. As CyberAngel mentioned, the one on top of her head does burn.
That's my 2p.