A model with a relic will automatically pass the first Leadership test he is required to make in the game. If worn by the leader, it will allow him to automatically pass the first Rout test if he has not taken any Leadership tests before.
Dogma dictates and enforces that only a single Relic can be taken into a fight per the entire warband.
Lore
In this age of superstition and religious fanaticism, holy objects are an important part of life. Relics abound in the Old World: hairs of Sigmar, pieces from Ulric’s hammer, teeth of Daemon Princes, all are sold to men needing encouragement before battle and as charms against sorcery.