Yes, I think that identifying the purpose would increase the credibility of the source, so if we understand the "should" here as "would be good if, is recommended" - then this statement is true.
Including the intent of the information in the source will help place the information in a wider context - and often the context is necessary to fully understand the content of a source.
Answer:
The key role of citizens in a democracy is to participate in public life.
Byzantine Empire: Greek was the official language, Eastern Orthodox Church, centered on Constantinople.
Roman Empire: Latin was the official language, Roman Catholic Church, centered on Rome.
Hope that helps!
