system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do we need/have different currencies and how can one be stronger/weaker than another?
gpt
Currencies today have value because a government lets you pay taxes with it and subsequently, other people will trade you stuff for it. Places with a lot of economic activity have popular currencies because they can be traded for a lot of stuff. There's also a supply/demand thing to it. Too much currency makes it less useful because you can't trade it for as much. So currencies differ in how useful they are to trade for things. The US dollar has supremacy in the world because oil producing entities mostly agree to trade with it, so everyone needs USD.

We have a bunch because every country basically has its own. It serves as a tool to control the economy and credit markets. Lots of currency makes it easier to make loans, but harder to buy things. You need a good balance. But you can't trust another country to manage their currency for another country's benefit. Big exception being the Euro, a bunch of countries agree to use it and have a bunch of rules about how it's managed.