The formation of gangs within prisons around the world.

What is the purpose of having prisons in Brazil? Due to corruption within the correctional system, prisoners own cell phones and are able to terrorize the whole city of Sao Paulo, which is the second biggest city in the world! Here is a link about gang's formation within prisons of Brazil, China, South Africa and El Salvador.  El Salvador, is the smallest country of Central America, but the most populated-over 5 millions inhabitants. How can "las maras" gang from El Salvador are terrorizing the whole country, and affecting their neigboring countries in Central America, uncluding areas in the United States and Mexico?

http://www.pbs.org/wnet/wideangle/uncategorized/18-with-a-bullet-brazil-primeiro-comando-da-capital-pcc/2242/