Monsters - what are they for?

Monsters of one sort or another are a staple of adventure stories and role-playing games - from Tolkien's orcs and Heinlein's arachnids, to Sauron or the Joker, our heroes need adversaries.

So what are monsters for?
In this post, I'll look at monsters purpose in the story, and in the game.

Aside: Is story important in adventure games? 

Yes - we tell stories in our role-playing games, sometimes even despite ourselves.
Even in the war-game roots of the beginning of RPGs, the idea that a scenario is mutable by the actions of the players inevitably provided a story - at least an anecdote - that we could tell after the game.
Whether we set out to or not, those player characters grow and develop, and the bad guys become more cunning.
I've tried to run purely episodic games for club venues, where the players come and go - but the adventures naturally flow into each other. Players look for connections and stories. It's actually hard not to end up with some continuity between games.
A story is part of making a believable frame for the adventure - your characters could just beat up everyone, but despite how awesome that sounds at first, it ends up being a bit empty.

So if we're resigned to telling stories, what is the point of the monster in the story?

Firstly, let me clear up what I mean by "monsters".
I expand the definition of monster to include all belligerent adversaries - it's a gaming term: "monsters" are what heroes fight. So monsters include Stormtroopers and Darth Vader, orcs and Nazgul, and so on.

Already we can see that we've got more than one type of monster role: Vader is in a different league to mere orcs. Also it's fair to say that Stormtroopers and orcs may fill similar roles, but they are of different types.
I can identify three major roles for monsters, and four major types:

• Roles: mooks, lieutenant, Big Bad Evil Guy/Gal (BBEG)
• Mooks - the TV Tropes link there explains the role of the mook very well, but to quickly summarise: these low level cannon-fodder are there solely chasing and driving the heroes on, and letting the heroes swashbuckle. Without mooks, your action adventure has less in the way of adversarial action.
• Lieutenant - or "The Dragon" in that TV Tropes link. This adversary is the arch-enemy's main muscle, the force he sends to attack the heroes. The lieutenant often leads mooks, and is rarely fought directly, except as a climactic fight
• BBEG - the cause of the evil plot. This villain is a smart, resourceful adversary, but often physically weak - the archetypal wizard. It is usually their scheming that makes them an adversary, not their combat actions.

• Types: horror, criminal, totalitarian, natural
• Horror - these monsters are unthinkable, driven by unknown or mad desires - especially when those motives, once revealed, are frightening or transgressive. The eponymous Aliens, the Thing, Romero's cannibal undead, vampires (when they're not being sparkly or emo) - these are all monsters of the horror type.
• Criminal - this type not only includes the expected bandits, burglars and buccaneers from that link, but also adversaries out for personal gain (or for their clan or gang), with amoral attitudes and a lust for power or wealth - including corrupt clergy, power-hungry politicians and the like. One of the more human monster types, and thus one of the most varied types.
• Totalitarian - the monster wishes to enforce its views, rules, or way of life on everyone, or destroy them in the attempt.Unorthodox behaviour will be crushed. Minorities will be swept aside. You will be assimilated. Includes the Machines in the Matrix and Terminator, the Empire in Star Wars, and various real-life regimes.
• Natural - no link for this one: it's a broad category, which includes many aspects of normal behaviours... just the kind of normal behaviours that puts a creature at odds with heroes. The monster becomes an adversary out of instinct or natural reasons. Think of ignorant mooks led by BBEG's propaganda, a shark feeding on tourists, or natives reacting to invasion or settlement of their homeland. These monsters may even be sympathetic adversaries in more enlightened stories - players may wish to at least try to defuse the conflict without violence.

Why do you need to know this?

Knowing your monster's niche tells you how it should behave, and what it does for the story. It's the first step in playing the role of that monster.

A criminal mook is clearly different from a horror mook, although they may serve similar roles in terms of action in the game - both line up to obligingly die at the heroes' hands. A horror lieutenant is different from the horror BBEG - spoilers here: Kiefer Sutherland is not the BBEG of The Lost Boys! The big reveal is that he's just lieutenant to the Head Vampire (who would have got away with it too if it wasn't for those meddling heroes).

So assigning the roles to your adversaries creates the beginning and hints of a story for you, too. Someone is usually in charge of those mooks - who is it...?

Before you know it you've got enough bones to hang a plot on.