The messages carried may be subconsciously and subtly conveyed, but the process of repetition [… ] consolidates the delivery of the message. 
? Jack Child, Miniature Messages: The Semiotics of Latin American Postage Stamps, 2008.

Sometimes nations coordinate the imagery and issuing dates on their postage stamps to support common causes. These international projects can provide financial backing as well as visibility to campaigns for humanitarian aid, public health initiatives, social services, and other shared concerns.