Why do different countries have different currency? Why do different countries have different currency? 


 I have this question because I want to know why to divide it? If it's not divided then we can easily use the money anywhere and we won't need to use currency exchange.
Two reasons: history and ability to control the money supply.

History: paper-money started by being issued by trustworthy agents from kings/governments, as a way to be able to transfer the ownership of gold without moving the gold physically. This looked something like "This paper is exchangeable for 10 gold coins at this location". This meant you had to go to that specific kingdom to get the paper (bill) converted into gold. These papers evolved into what we now call banknotes, and kept tied to a specific country.

Money supply control: having your own currency allows governments to fund themselves by producing more money, or stimulate exports by devaluing their currency. Look at the euro-zone difficulties with the government debt of Greece: this could be solved (with consequences, of course) if they had their own currency.