Quote Originally Posted by Reverent-One View Post
Or since the book's purpose is to "Give the DM things for the PCs to fight", that it's giving the understandably relevant details of why the PCs would be fighting them to the DM as well.
Except that's sort of the job of the DM's world-building and plot, as well as the PCs themselves to determine. A well-designed campaign doesn't at all hinge on the monster manual painting a target on pretty much every entry it has. It sets up reasons beyond "Splatbook says" for conflict and plot to take place.

Unless, you know, your campaign is essentially a series of rooms with a fight in each. In which case, yeah, I suppose the DM could use a canned excuse to toss whatever encounter looks interesting at his or her players.