The tango in Argentina is a prime example of this. The dance began in the slums in Argentina to express the heart ach and hardships of the poor people. The dance spread among the classes and now is a symbol of Argentine national identity. This one dance was able to break the social differences and create some sense of unity. Another point that I don't think was mentioned, was censorship.
Art is a weapon that can be used to give the people power.
Some governments try to eliminate this weapon through the use of censorship.