Governments are known to take part of rationing during hard economic times or war. During extended periods of rationing, black markets are often created to supply consumers with the controlled good.
For example, during World War II, the U.S. government imposed rationing on the country so that sufficient materials and production capabilities would be available to the military.
Rationing can lead to the creation of black markets for the rationed goods. Black markets allow individuals to use their allotment of a rationed good that they don't need to obtain more of a rationed good that they do need.