Cash is a weapon in war, always has been. The ancient Chinese philosopher noted that the easiest way to capture an enemy fortress was with one mule (loaded down with gold, to bribe the fortress commander.) Some countries, or rebel movements, have found that if they can't get cash by donations, or theft, they can just make their own. Thus counterfeiting U.S. hundred dollar bills is a viable way to get lots of cash. But as the U.S. has changed the design of its currency, to make it more difficult to counterfeit, there are fewer players capable of rolling their own hundreds. At present, less than one hundredth of one percent (one in 10,000) hundred dollar bills out there are fake. Most of them are in Asia and South America. Currently, the two major sources of this fake currency are North Korea and Colombia. In the case of North Korea, the operation is run by the government. This has been going on since 1990, and in that time, half a million North Korean "super notes" (they are very good) have been spotted and taken out of circulation. Many more were not caught, and are still in play. But the North Koreans are not the biggest supplier of this phony cash. That distinction goes to Colombia, where several criminal gangs currently produce 15 percent of the fake U.S. currency in circulation. The gangs are protected by drug lords and leftist rebel organizations. Because the counterfeiting operations are so small, the they are difficult for the government to take down. This is especially the case when the drug gangs and rebels are a larger threat, and thus a more important target. Nevertheless, about half the fake U.S. currency seized each year (over $50 million) is found inside Colombia itself. Only about 15-20 percent of the fakes get grabbed in the U.S. As a practical matter, the Colombians and North Koreans find it easier to pass the phony bills outside the United States. Typically, the North Koreans or Colombians will "sell" their counterfeit bills, as such, to others, at a discount. Thus, in out-of-the-way parts of the world, where there are fewer people who can spot a good counterfeit, the bad bills become a secondary currency. In many countries, U.S. currency, even if it's possibly counterfeit, is seen as more reliable then the local stuff.
In wartime, it's a common practice to counterfeit the enemy currency, so as to have enough to support resistance movements, or bribe enemy officials. These days, fake U.S. currency is seen as a universal medium for this work.