Celestial Bronze and Imperial Gold are actually just two common material for these reasons:

- Celestial Bronze is what allows a weapon to b disguised as another mundane item.

-Celestial Bronze is relatively lightweight and easy to train with, as well as fight.

-Imperial Gold is imbued with Godly properties, often only used for the rarest of weapons.(Spear of Destiny, Poseidon's Trident, Zeus' Thunderbolt.)

-Imperial Gold also possess a sort of "teleporting" effect. (I.e. This is why you see tales of Weapons appearing before a great hero when he needs one the most in his travels.)

Any form of blade can damage the beast, but Celestial Bronze and Imperial Gold have unique qualities. Silver, as it has been said for many thing throughout history and mythology, has always been said to have properties of it's own. I am sure everyone knows the more common stuff, such as Lycans and Vampires and other similar creatures, but in Mythology Silver is said to affect something different in each Religion. In the Greek and Roman Pantheon(Sister Religions would obviously have similar details.) Silver is said to be the best weapon to fight Apparitions, Spirits, Ghost, and other Spectral beings. 

*I even noted what Silver was good for in my Bio.*