Prayer armour is not only meaning for protection, but lowering the speed at which prayer points drain, which can be helpful in some battles.
Prayer armour is more diverse in terms of types of armour available, including items from all three forms of the combat triangle, generally offering decent, although not outstanding, defence. A large number of Prayer armour, similar to Magic armour, is however more in the line of robes, although it is this category that often gives the highest bonuses.
Most Prayer armour in Runescape is only available for members, such as Proselyte. Exceptions include Monk's robes which can be obtained at the monastery and are accesible by both free gamers and members.