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.