This can be quite useful as it allows you to train and use units from other faiths. You have your choice of 8 faiths to play. Each faith has distinct unit types, magic spells, and cities. The choice you make will greatly impact the way you need to play the game. For example, Order has powerful in close melee troops and weaker, relatively short range missile troops.
Life has exceptional, long range missile troops and very weak melee troops.
Your battle tactics are thus greatly impacted by your choice of Faith. You are also faced with choosing a class for your Lord. You can choose to be a Fighter, Mage, or Thief. This isn't as vital, since after liberating your temple you'll also get one of each other type of champion. Still, it has a pretty big impact on the early game before the temple is freed.