Leather Armor is a more robust type of armor, it offers decent defense and it's not very heavy. It can be worn by the Warrior and the Conjurer and the Leather Boots are the only type of leather armor that can be worn by the Wizard. The Conjurer Helm can only be worn by the Conjurer.
For conjurers it's the best protection since they can't wear heavy and clunky plate armor as it difficults their swiftness and conjuring.
For wizards, the leather armor is strictly forbidden as it is against wizardry code and practice because it difficults spell casting and makes the wearer more slow and vulnerable for opponents.
They use Cloth instead.