Before the invention of mass media, stories were spread by virtue of their impact.
The more awe-inspiring and transformation-inducing the tale, the greater the likelihood that the story would "go viral". In other words, the transformative value WAS the market value. The very life-blood which determined which stories lived and died.
Through continuous telling and re-telling, the collective unconscious of society engaged in a dialogue with the story material itself, and thus produced works which stimulated the development of the kind of individuals which were most needed at any given time.
The Greek heroes embodied courage and will power at a time when the world was still a wild unknown. The romances of the 18th century called forth a new level of emotional intelligence as a response to the brutality of the Industrial Revolution.
In the modern era, this dialogue between the storyteller and the collective unconscious does not always take place. It is possible to create, market, and sell something that appeals to the masses yet has no lasting impact on the audience or society.
Nevertheless, the biggest pop culture phenomena are still those which express the otherwise unconscious archetypal truth of the times.
This is the impact that every story and storyteller secretly aspires to.