I was never able to figure this out. What exactly is the point of silver weapons and armor? What sort of bonus do they give?

Some monster types immune to normal weapons are vulnerable to silver, but magical weapons are better. Silver shields can mirror gaze attacks by medusas and other monsters.

another fun use of silver weapons is silver arrows can be used against wights where a normal arrow cannot, giving you a chance to take out the wight "leader "that can appear in some of the random undead groups when they first appear at a distance

Silver weapons harm the undead but have no bonus