Everything humans create isn't worth anything except to us humans, we're the ones who put value on it. 

Correct, that a fact, that the bottom-line. But we all value things not the same, as you may value Beanie Babies, I see it just toys that worth nothing.

For example, a tin opener - 99.999999...% of the universe (or even animals on Earth) doesn't need a tin opener but us humans create them and sell them to each other for a couple of (insert local currency). It is worth something to us because it makes our lives easier. 

Not great example, but we human created the CAN, and created the CAN opener, while could just incorporate seal can be pulled to open the CAN without the CAN opener. Reason is cutting cost on that "incorporation" of making it pull tab, saving them few cents per CAN, this scale by the X sold of CANs to Y amount you save, example 5 cent a CAN, save $1 every 20 CANs sold, $10 every 200 CANs, $100 every 2000 CANs and you get the point how cutting corners works. Marketing finds away to sell a product people want, or need to boost sales.

While humans find ways to make life easier, or solve problems, it doesn't mean humans won't find ways to make profit off something, or others.

You can apply that same logic to anything.

For millennia humans have tried to find a way of trading across borders and languages, whether it's silk or metal or food - hundreds of times we've created systems that try to make this easier for us. Whether it's by creating silver and gold coins or currency or simply agreeing that 1 cow = 1000 potatoes. 

However, time and time again these systems have failed. Firstly, the cow for potato has failed because one year it doesn't rain as much so potatoes don't grow and therefore there aren't as many to sell, so now 1 cow = 10000 potato. With gold and silver coins, a new deposit is found and the amount in existence grows which makes the existing gold coins worth less because someone can just mint more. With currency this is literally happening every minute - governments/banks are creating debt to create interest which means the amount of money in circulation goes up which means your money becomes worth less. It's simple economics.

This is why we MOVED away from old barbaric bartering to creating currency in it place to make it EASIER to bartering to help maintain some kind of value in those items, or ease off on drawback. See we can go back to trading potatoes, but it ROTS, and doesn't last long, while cow can live for many years. Money is something that doesn't rot, or age, it only created as value for bartering. While value of things fluctuate, it can be due to inflation, SUPPLY & DEMAND, taxes, tariffs, and so on. Even with old barbaric bartering it still had issue with inflation hence if had bad crop year, you raise that value what is worth, and say NOW 1 cow = 700 potatoes, or one year when grow too much 1 cow = 1200 potatoes, so get the point. Yes, when have more of something made the less value it is, the less of something made, the more value it is, that simple.