The USD is a pretty lousy global currency. You can't exactly use it anywhere in any sort of transaction. I've only ever heard of a handful of places outside of the States where the local currency is terrible enough that local people prefer to hold and transact in USD over long periods of time. A true global currency would be one that you could use to buy practically anything, anywhere, whether it was a very large or very small transaction.

Regardless, Bitcoin has several advantages. For one thing, you're not dependent upon continually obtaining a flow of paper notes from a foreign country that may lie halfway around the world. For another, you can divide Bitcoin to extremely small and precise quantities - the dollar on the other hand doesn't scale as well to economies where a penny may start to approach a significant individual unit of money.

There are of course many other advantages of Bitcoin that make it attractive for people in many different countries to adopt. The fact that you can transmit and receive it to and from anywhere in the world, with the same certainty of transaction confirmation as cash in hand, without the need to either physically ship cash or rely on a network of physical cash shipment, is a pretty good one, especially as the world grows ever smaller.

A final advantage would be that you are never hostage to the political dangers of a nation-state controlled currency (i.e. a small group of rich white men can at any point decide to devalue your savings, to the point of destruction, by printing an unlimited amount of the currency.)