Rules matter. As long as you rely upon them to generate results, they guide play in various directions by encouraging some choices and results while discouraging others. Vince Baker, one of the most insightful new-school game designers, likes to discuss this in terms of “emergent play“: results that emerge from unexpected interactions within—and with—the rules.
In old-school D&D, the most significant example would be the advancement rules. By the book, most experience points come from obtaining monetary treasure. This encourages to prioritize treasure-finding over other activities, and discourages adventures that yield no monetary profit. Defeating monsters serves as a secondary source of experience points. Low-level monsters give a lot more experience in White Box than in Red Box; in actual play, we’ve found that the players in Tavis’ White Box game are more prone to seek out fights without a guarantee of monetary reward than in my Red Box game, where the party generally eschews combat if financial profit seems unlikely.
As Philotomy has noted, a lot of old-school gamers stopped giving out experience points for monetary treasure because they felt it didn’t make sense in the context of the fiction. This alters the play of the game! Seeking out treasure in emulation of swords & sorcery heroes like Conan or Fafhrd and the Gray Mouser is an emergent behavior derived from the XP-for-gold rule; without that rule, play tends in other directions as players seek to gain experience and level up their characters. My own history as a player bears this out; in a favorite 3e D&D campaign where we gained no XP for treasure, we’ve actually turned down monetary rewards because that seemed appropriate to our characters.
The lesson to take away here is that, when designing or modifying rules, it’s important to consider how the rules guide behavior! As demonstrated above, putting verisimilitude above all other considerations can lead to unexpected—even undesirable—effects on play.
When I fleshed out the Vancian magic system in my setting, I was concerned with encouraging certain elements of play and discouraging others. Defining the rules around these concerns was my primary goal; the flavor and color, however important, were subordinate to these considerations.
First, I wanted to strike a balance between allowing magic-user PCs to share useful spells and retaining their specialties. On the one hand, everyone wants access to powerful spells like sleep and fireball. On the other hand, a magic-user’s spell selection provides a colorful distinction from other PC magic-users, much as how fighters are often distinguished by their magic weapons, or how any character might be distinguished by a unique special ability. So I split the difference between making spell sharing free and denying it altogether; researching one’s own version of a fellow PC’s spell is not instanteous or free, but provides a discount over designing a spell from scratch. (This dovetails with our carousing rules for turning gold into XP; learning new spells, whether from fellow PCs or through original research, always generates experience points.)
Second, I wanted to give access to new spells in a controlled fashion. I wanted the ability to give access to new spells as part of a treasure hoard, but I didn’t want the PCs to immediately gain access to every spell in a defeated magic-user’s spellbook. So I engineered the system to allow me to control how many new spells the PCs received by defeating an enemy M-U. Ideally, this will give the magic-user PCs a nice benefit from captured spellbooks without encouraging them to spend all their time hunting down and killing their NPC counterparts to steal their power.
It’s only at this point in the process that color considerations come in. But even color considerations have emergent effects! For example, requiring initiations with expensive components to learn new spells has a number of effects on the game world. Wizards’ labs will generally be full of expensive components, and that’s treasure. Magic becomes more exclusive; since you need a lot of money to become initiated into the art, only the wealthy or the chosen apprentices of existing wizards can afford to practice magic.
On the other hand, one can base magical traditions around cheaply procuring components that are expensive for city-dwelling magic-users. Shamans would gather their own herbs and minerals, for example, while dark wizards would exploit the price that slavery places on human life by performing initiations around human sacrifices!