To my mind, money in an RPG is a tool to facilitate any or all three of:
- Score-keeping behaviour (numbers go up big! Yaaay!)
- Giving the players interesting decisions to make.
- Helping the world feel lived-in, alive, like a place that exists independently of the gameplay.
The more you want to emphasise
score-keeping, the less you need to worry about how much money the player characters accumulate. One way of score-keeping is the old-school GP-for-XP mechanic.
Emphasising
decision-making can involve different proportions/combinations of
restricting the amount of money the player characters have or
providing suitably large money sinks, and reasons why player characters might sink their money into
this money sink instead of
that one (or vice-versa).
World-sim is probably the hardest possible use for the idea of in-game money, especially if you're making it a priority, since that's where you start having to try to emulate a functioning economy with game mechanics - and good luck with that! That said, I am sure fans of world-sim play can probably point to games that do so well enough for their tastes, and indeed a few examples were given on the first page of the thread.
D&D
de facto settles on score-keeping, but the wildly variable costs of items, as well as systems such as Bastions, give it the veneer of catering to decision-making or world-sim. Since most game designers are not economists - and for most players, there isn't the emotional or gameplay payoff that justifies the effort to create "realistic" or "verisimilitudinous" in-game economies - it doesn't surprise me that most games also emphasise score-keeping or decision-making over world-sim.