system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't all countries have the same currency?

I mean other than prestige and uniqueness, what's the point of having different currencies?
gpt
This comes down to what is known as optimal currency areas (or optimal currency union). One main issue is you can't have regionally different monetary policies under a single currency. Consider the US and EuroZone, there 1 monetary policy. A  key characteristics of a optimal currency area are the same exposure and response to shocks. That is if one area is in expansion all the others are as well. The mobility of labor and capital are also important qualities. Suppose you have a region on recession and one in expansion what should the central bank do? Expansionary policy for the recessed region? Restrictive policy to slow the growth in the growing region? 

One way to account for this is a fiscal union. This is one reason the US is considered an optimal currency area and the Euro isn't. The US federal government can use fiscal policy to balance out the regional differences, where the EU has less ability to do so. So absent a world fiscal union having one world currency would not be a good idea.