When a doctor walks into a barbershop why does he not offer his medical services as compensation for a trim?
When a farmer needs to fuel up his truck why does he not offer some of his excess harvest in exchange for gasoline?
These are two examples of potential barter. Barter is the ultimate peer-to-peer interaction. No third parties - just direct exchange of value. People still do this occasionally within a community. I can negotiate to watch my neighbor's home while he travels to South America in exchange for him returning the favor when I go to Europe.
You may already see the problem. All of this disintermediation comes at a great cost. The double coincidence of wants is the name for the observation that what I want and what my counterparty has to offer rarely align perfectly. Even in the house-watching example my needs differ from my neighbor's across time. Meaning that if I watch his house I have to incur the additional risk that he may be unavailable or unwilling when I call upon him.
Humans are clever. We make technologies that make us more productive and our lives more enjoyable. Money is one such technology. Money solves the double coincidence of wants because it serves as a store of value that many people agree on. Like many tools - money is useful.
I have an idea for where I want to take this next but I am interested if anyone else has thoughts to this point they may be willing to share. Who knows - together we may get a result none of us anticipated. This is interactive community learning of the highest order.