There are a couple of main reasons (although many more) why most countries have their own currency:
1. Each country wants to be able to regulate their own economy, having their own currency that they control gives them that power.
2. It provides a small barrier between a nation's economy and the global economy. An example of this is Greece with the Euro, though luckily it was contained to Greece as far as I know. If a nation's economy collapses and they have a worldwide currency it could be very bad.
Indeed. It also gives a country its own identity different from other countries. I think having one currency would just to more division and power struggles between rich countries and it may very well lead to military conflicts as to should have control over that one currency