After you have built a Temple or Unholy Temple, you can assign creatures to begin praying in it. The room does not need to have a sacrificial pool in it; in fact, you might find it advantageous to make it without one, to avoid having creatures fall in accidentally.
Praying in a Temple will protect a creature from being scavenged, and will increase its Happiness. It can also be used to cure your creatures from being turned into a chicken and to help protect the spread of disease. This can be exceedingly useful, as it can quickly turn an angered horned reaper into a calmer state, saving it from rampaging through one's dungeon.
Dungeon Keeper 2Edit
Expensive as it is to build, an Unholy Temple can end up coming in very handy. Any kind of creature can pray in it, and not only does this keep them occupied and out of the Casino, but it also heals them and generates mana for you. The amount of mana generated per turn depends on the type of creature:
1 Does not include doubling from being converted.
Vampires have the advantage of praying automatically, if there is nothing to research and there is room for them at an Unholy Temple. They will pray even to the exclusion of training, which is handy as you do not want them to train up to level 4.
Despite their relatively low mana output, a horde of Skeletons can be a good source of mana if you can get them past level 4 (or put them in the Unholy Temple before they train up to level 4), because they will literally stay there and pray forever until you make them do something else. You can lock 24 of them in a 5x5 room, all filled with Unholy Temple except for the center square, and have a constant stream of 720 mana per turn.
Another nice thing about having the undead pray; if they are injured, praying will heal them. Since Skeletons do not sleep, this is a handy way to keep them at full health without having to use the Heal spell.