In the Northwestern Isles, the younger sons of the nobility can either resign themselves to becoming the clients of powerful nobles or they can range the wilds, protecting the villages and forts from invaders both natural and unnatural. To become such a protector, they must join initiatory warbands. There are several types of these, but the most well-known to the rest of the world are the werewolves.
Joining up with one of these bands, a young man (or even a young woman, if she wishes to leave behind the life of a wife and mother) will spend some time traveling with the others, learning the nomadic way of life of these wilderness warriors. During this time, they will learn to fight, to live off the land (and, frequently, off of those poor unfortunates who cannot protect their property), and the secret lore of the werewolf bands. After a time, perhaps a year, perhaps three, they will undergo initiation as a werewolf. In a secret rite they will be transformed from a mere human being into a wolf-man, bound to the cycles of the moon instead of (or, rather, in addition to) the natural daytime cycles of men.
Though folktales say that a wound from a werewolf will make one into a werewolf, that is not true at all. Only the secret initiations of the werewolf bands can make one into a werewolf.
Once initiated, a werewolf will spend a lifetime protecting civilization from invaders. Though the world is mostly aware of the human invaders, it is the inhuman ones that the werewolves are most interested in. Goblin hordes from across the Mist between the worlds are constantly making incursions and stealing livestock, crops, and even children! The werewolves stand between the world of men and these invading goblins, sometimes passing between the worlds themselves to recover the booty taken by the goblin thieves.
There are similar groups in other parts of the world: werebears, wereboars, and weretigers being the most common, but some lands have werefoxes or even weresharks. Those lands without shapeshifting guardians may have other ways of protecting themselves from goblins, and even in the lands with shapeshifters, there exist other initiatory groups who stand on the border between worlds.