No Stupid Questions

People want purpose. Most people are too inept to grasp greater purposes, so rewarding them with ornate paper slips keeps them placated.

Because it gives the ruling class something to hold over the poors so the rest of society is so terrified of becoming poor they forget that they're being used as literal slaves for the state

Who cares about a return on one's effort amirite

Life ain't nothin but bitches n money

Those are 2 very different questions

Here's my take:

1. We're built for about 150 relationships max (Dunbar number), and yet we benefit from cooperation above that threshold. Rather than make it so we have to have a personal relationship with everyone who could possibly benefit us, we accepted a ramped down version of relationship we call "transactions". This is a very weak replacement for a relationship, but it is a sort of "micro-relationship" in that for a brief moment two people who don't know each other can kind of care about each other during an exchange. Through specialization, we can do something well that doesn't just benefit the handful of friends and neighbors we have, but tens of thousands and possibly millions of people via transactions (e.g. a factory, starting an Amazon business, etc.)

2. There is a process called "commensuration" in the social sciences, where people start to make one thing commensurate with another, even in wildly different domains. For example, to understand the value of a forest and to convey its importance to decision makers we might say "this forest is worth $100 billion". It's kind of weird to do this (how do leaves and trees and anthills and beetles equal imaginary humoney?) But slowly, over time, we have made many things commensurate to dollars at various scales. (I don't think this is a good thing, but it does have benefits). In short, more and more things that were part of an implicit economy of relationships (e.g. can the neighbor girl babysit tonight?) have entered the explicit domain of the monetary economy (e.g. sittercity).

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IMO, in order to participate in the huge value generated by this monetary economy, people sometimes lose the forest for the trees (so to speak) and forget what really matters (e.g. excellence of character, deep relationships, new experiences, etc.) because it seems like we might be able to put off those things until "after" we square away this whole money thing first. But maybe "after" never comes--and the hollow life of a consumer capitalist drains the inner ecological diversity of a soulful life.

Maybe it's not as essential as one is led to believe. The book Debt by David Graeber deals with the very question you asked. I highly recommend you check it out.

I can't really summarise it well since I haven't finished it myself, so maybe peruse the Wikipedia entry. But the first couple of chapters try to answer your question

https://en.wikipedia.org/wiki/Debt:_The_First_5000_Years

There is also a meta discussion here on liquidity (how easy it is to exchange something). People want currency because it is easy to exchange. Your landlord might be okay with getting paid in a mix of gallons of milk and some cuts of beef, but most will not. Trading for "non currency value" is not uncommon. You might get paid with stocks depending on your work. If you buy a car you might trade in your old one.

Can be exchanged for goods and services

Currency is an energy equivalent in human society. I'd say our current ridiculously exploitable system with greedy for profit banks, rampant credits, tax havens, exchange rates manipulation, dividends and stock market micro trading leaves a lot to be desired, yet it is still incredibly convenient and relatively stable even with all the major crashes. Beats barter that's for sure.

We are focused on it because life itself is based on burning energy to sustain itself. Any plant would be happy to get more nutrients and sunlight like every animal would gorge itself to get some of that fat on their bones. It doesn't matter that it's sometimes to the detriment too - it's just wired that way in the DNA, because scarcity is way more common than abundance and if this balance crumbles then it's just a matter of time before abundance turns to even more scarcity due to overgrowth/desertification/overpopulation.