In the original game, you get (some kind of) points for slaying monsters and getting treasure; when you get enough of these points, your character improves. We call these points "experience". But then, we start thinking of "experience" as actually representing experience, in the sense of knowledge or practice. You're getting better because all those risks you are taking are adding to your knowledge of how to deal with problems. And once you've fallen into the trap of thinking this way, it looks ridiculous when you bring home a bag full of rare gems and old gold coins, becoming better adventurers as a consequence.
And so, you get rid of the experience for gold equation, because it's "unrealistic". Even though around the same time people ditched XP for GP, they added XP for "good role-playing" and story rewards, and penalized people who deviated from their alignment. How do either of these have anything to do with practical improvement?
Take a look also at risk. Supposedly, XP for GP is bad because getting the gold does not always represent the same degree of risk. Consider four potential treasures:
- Five rats with 1200 silver pieces, randomly rolled.
- A giant spider with 100 silver pieces, also randomly rolled.
- A treasure chest with 200 gold pieces, trapped with poison needles.
- 400 gold pieces hidden behind a loose stone in the wall of an otherwise empty room.
Clearly, if an adventurer tackles the spider instead of the rats, the reward is lower, despite the risk being higher. The trapped chest has an even bigger reward, possibly with more risk than the rats, but also possibly with no risk (Knock over the chest with a ten-foot pole, maybe, then collect the spilled coins without risking the trap?) The hidden treasure cache has the biggest reward and no risk; you get the reward simple for exploring an empty room.
This is the kind of thing that sends some people into conniptions, even though the same thing happens when you just consider XP for monster-slaying. That giant spider might be worth, say, 100 or 200 XP... but you get that XP whether you fight the spider face to face or just torch its webs. You can lure monsters into a trap and collect the XP reward, despite taking less of a risk. And, in fact, the original playtesters say that the goal of adventuring was always to get those XPs with as little risk as possible.
My point is that experience doesn't or shouldn't represent taking risks and learning something from the experience. "Experience" is not literally experience, it's either just a meta-level reward or it represents something "real", but not literal learning. The meaning of "experience" was muddied by some early rules incorporated into the game... but I'll talk about those in a later post.