you can give those plates + other staff like layline crystals at a vender when you kill the final boss at dragon’s stand and buy a weapon. some weapons need x2 some need 3. ( plated weapons. " plated Longbow " for example if i dont write the name wrong.
I know for sure that you need that weapon for the specialazation collection so you can take ascended weapon from achivment.
atleast for dragonhunter.