One oddity of AD&D 1st edition that had always seemed nonsensical to me, is to give characters XP not only for defeated monsters, but also for the value of treasures they bring back with them into town. Why do that? Picking up stuff that is lying around does not make you better at fighting or casting spells. And in those games I’ve been playing the most, treasure is there to be sold so you can buy better equipment and magic items. But in campaigns of a more oldschool leaning, there frequently are no more things for sale, which you don’t already have by 2nd level. So why bother with treasure at all?
Very often, and probably most of the cases, defeating an enemy also gets you treasure. But you can also defeat an enemy and not getting any treasure (because he doesn’t have any). And you can get treasure without defeating an enemy!
That’s what makes XP for treasure relevant. Sometimes an enemy can’t be fought, or the risk is regarded as just way too high. But if you can find a way to get his treasure while avoiding him entirely, you still created a clever solution to a problem. Which is rewarded with XP. Even in a game where money has no practical use, treasure still serves as a measure of your accomplishments. When you return to town, the treasure you bring back with you is your proof for your deeds.
You can’t make the player feel the comforts the money of the PC can buy him. And it’s extremely difficult to really play out the benefits of good clothing and a fancy house. To the character, being rich has great value and benefits. And when the chracter sees a golden idol, it is luring him with expensive wine and crocodile skin boots. But since comfort does not carry over to the player, XP can serve as a substitute lure. Instead of dollar signs in the players eyes, it’s saying “XP”. What matters is the emotional response.
When the GM describes a golden idol with ruby eyes on a pedestal, the player should think “I really, really want this. I hope there’s a way to get it without getting killed.” In other games like D&D 3rd edition and later ones, the player will want to have it because it can be traded in for magic boots or enchanted armor. So there is no need to add the additional lure of XP.