Temple is a village building and the job site of the cleric villager, who uses a brewing stand as their job site block. It is not a desert-only building: the game ships a temple for every village style — desert, plains, savanna, snowy and taiga — each one built from its own village's materials.
The desert version is the one that also generates a loot chest, and only one of its two layouts has it. The chest can include items such as bread, rotten flesh, redstone dust, emeralds, lapis lazuli, and gold ingots. Bread and rotten flesh are the most commonly found items, while gold and emeralds are less frequent.
Every version of the building contains a brewing stand, which is what makes it the cleric's workplace and one of the few places a brewing stand generates ready-made. Temples are part of the random pool of village buildings, so not every village has one.