In a past life, I played a lot of Gwent, the Witcher-brand digital card game that grew out of the minigame in the Witcher 3. It's good. It's got lots of interesting builds and decks, the balance hasn't been truly terrible in years, and the card art is phenomenal.
In Gwent—and other card games—there is a strategy called "bleeding." When you bleed, you play shitty cards from your hand on your turn in order to extend the round, thus forcing your opponent to play good cards. You're already planning to lose the round as is, and you don't mind throwing away your bad cards, so any card you force out of your opponent this round is one you don't have to deal with next round. It's a classic strategy, really.
There's also a game mechanic in Gwent called Bleed. Bleed applies a counter—usually 1–6 or so—onto a card, that saps 1 HP from that card every turn thereafter. Bleed has its purposes: it's not good in every situation, but lots of decks rely on it for various strategies. It's damage over time.
Bleeding, the mechanic, is quite useful in decks that try to bleed their opponents. Slow-ticking damage over time gets a lot of value in long rounds, and cards that deal Bleed damage (as opposed to normal instantaneous damage) are usually pretty cheap. It's a fun resonance, especially since bleeding, the strategy, predates the Bleeding, the mechanic.
One of the factions in Gwent is the Monster faction. One of the many possible decks that Monster players can run is a Vampire deck—you cram a bunch of Vampires into your deck, some engines that key off of Vampires, some combo pieces, and one or two bits of tech to fill int the gaps. It's a classic.
Because the designers know their lore, Vampire cards often deal Bleeding damage. Likewise, lots of the Vampire engines require enemies (or allies) to Bleed, and lots of the Vampire cards require long rounds to be successful—even the cheap ones. You see where this is going?
Vampire decks bleed their opponents dry.
It's a brilliant piece of design. The mechanics serve as the perfect counterpart to the theme. You feel like a vampire when you play a Vampire deck.
Traditional game design like this, the kind you see in most games, is mostly an act of metaphor. Designers take some non-game thing—warriors fighting, nations building, birds nesting—and make it into a game. Games (usually) are composed of mechanics and systems and rules, and clever design requires making those rules convey the feeling of the non-game, sometimes better than the original non-game object could on its own.
This metaphorization isn't unique to games: all media does it, to some extent. A Dutch angle conveys something unsettling or ominous because it feels strange and wrong, not because literally tilting a video is scary. Much of the work of artists and designers is to convey a specific feeling through a medium not ordinarily used to convey that feeling.
Because games are systems, though, they usually have to deal with layers of abstraction. Players don't want to wade through every possible level of depth and density, so mechanics are abbreviations, simplifications, zoomed-out views of a larger whole. You don't model each and every cell of muscle breaking under a sword-blow, you just lose some Health. Once you've reduced muscle and blood down to Health, though, that muscle and blood ceases to exist in the game as a whole.
Part of what makes traditional games good, then, is when even through this abstraction you still feel the original—you feel via the metaphor of game design.
Roleplaying games, I think, defy this.
Obviously, almost all RPGs use some level of abstraction, in stats and dice rolls and so on. But those abstractions still have a concrete reality beneath them: the diegetic world supercedes the game rules. Rulings exist because the rules do not and cannot cover every eventuality: RPGs are open systems. When the rules don't match the world, you change the rules. The metaphor doesn't need to exist because the reality is not being removed.
Abstractions and mechanics exist in RPGs because real life (or, you know, "real" life with dragons and lasers or whatever) is too complicated to model. You usually wouldn't, but if you wanted to, you could probably come up with a table that matches your Strength score to cubic inches of muscle mass in each muscle group, and then correspond that to how much you could deadlift with various groups. We don't do this, obviously, because it's a pain—not because muscle mass doesn't actually exist in the diegetic world of the game.
Here's a counter-example: in Monsterhearts (good game, Avery's a great person), there's a mechanic called Strings. Strings represent one character's power over another, basically—you "pull their string" to get them to do what you want, usually. It's a kind of social currency.
Every monster in Monsterhearts is a metaphor, just like (most) monsters outside of Monsterhearts. Each represents a different kind of toxic, broken, unhealthy, or struggling relationship. The Vampire in Monsterhearts is about abuse, specifically one person being in a domineering, controlling position over another. Here's one of the Vampire's moves:
This is, from a traditional perspective, pretty good. It keys into the traditional fantasy of playing a vampire (requiring an invitation), and it ties into the abusive-overlord angle, in that someone inviting you into their home grants you power over them—which, in some cases, is the classic behavior of an abuser. Monsterhearts is full of good metaphorical design like this, where the mechanics map cleanly onto the fictional story being told.
Here's a question, though: what does this move actually mean? What happens when you do it? You enter someone's home and, according to the game rules, you have a String on them, you have power over them: why? What's actually happened in the world of the game to cause this to occur? I genuinely don't know. I can kind of vaguely infer it—the vampire takes in details of their house and thus better understands their victim's psychology, or something?—but I don't understand the concreate (fictional) reality of what's occurring when this move triggers.
This is game design running away with itself. RPGs don't need this kind of ludic metaphor because they never lose any of their diegesis. This move (and lots of others) has lost sight of land: it's come ungrounded from the reality of the game world in order to perfect its own metaphor—a metaphor that's being used to describe that selfsame game world.
Now, obviously, there are some reasons to design it like this. The relationship angle is one: if you remove any pretext of the Vampire class, this move basically says "abusers gain power over individuals who seemingly-willingly invite them into their homes," which is a not-unaffecting thing to say.
But here's the thing: the metaphor, of vampire-as-abuser, already exists in the game world. When you meet a character that controls you with its gaze, simultaneously allures and horrifies you, and seemingly has infinite power, you are meeting a character with (some of) the traits of an abuser. You don't need mechanics to do this, because roleplaying games exist outside of their mechanics first.
Metaphors in RPGs exist because metaphors in writing exist, just like in real life.
There are lots of reasons for the popularity of this kind of ludic metaphor. One is that the games most people are familiar with, video games and board games, use metaphor like this all the time. If you're trying to adapt, say, Dark Souls to an RPG, you'll probably end up looking at a lot of video game abstractions and metaphors. Even if you're not doing an adaptation, you absorb the game design of the games you play via osmosis, and that comes out in your RPG writing. Mechanical metaphors are everywhere in games, so they feel natural to include in RPGs.
Another is that, as a designer, it feels good to come up with these ludic metaphors: it's hard work, transplanting fiction into mechanics, and so when it comes together, it really sings. You create these beautiful interlocking systems that perfectly express some fictional element, and that's very satisfying. (There's also a little bit of self-fulfilling loop here, where game designers want to feel relevant and important, and so write every increasingly-tight systems with little wiggle room for players.)
Another reason is that Vincent Baker keeps writing articles like this.
It's also that this kind of thinking, that ludic metaphor is inherently useless, leads down some strange paths. (Why do we abstract anything in RPGs? Why do we have mechanics? Why aren't manuals just writing about what things are, free of rules? Is this just improv? Does game design exist?) While I personally have fully accepted that brainworms have entirely eaten my designer-mind, it's been a long and rocky road to get here. If you're really used to designing entirely through metaphor (like I was), it can be pretty scary to just step out into weird writing-land where mechanics don't matter and the rules are made up.
But it's also very liberating. Nowadays when I write systems or classes or whatever, I just write what is true. Fighters are good at fighting. Wizards can do magic. I still use systems, obviously, but I don't try to convey messages and meaning through my mechanics. Mechanics exist because I don't want to describe through every swing of a sword and twist of a knife, I just want to roll 1d6 damage and move on. The rules exist to quickly and usefully gloss over everything you don't want to focus—they elide, as Jared would say.
I obviously still worry about metaphor, but now I worry about it like a writer. And, when the game design itch takes me, as it often does, I go and design board games or card games or video games. RPGs are just different, you know?