The Puranas are the magnifying glass of the Vedas. The principles and rules of dharma that are briefly dealt with in the Vedas are enlarged or elaborated upon in them in the form of stories. A subject briefly touched upon may not make a deep impression on the mind. If the same were told as an absorbing story it would at once make an impact on the mind of the listener or reader.
The Vedas urge us to speak the truth ("Satyam vada"). How one becomes exalted by remaining truthful at all costs is illustrated by the story of Hariscandra. "Dharmam cara" (Follow dharma, live a life of dharma) is a Vedic injunction consisting of just two words. The importance of the pursuit of dharma is explained through the long story of Dharmaputra [Yudhisthira] in the Mahabharata. "Matr-devo bhava" and "Ptir-devo bhava" ("Be one to whom the mother is god" - "Be one to whom the father is god"): these two admonishments are enlarged on, as it were, through the magnifying glass in the story of Sri Rama. Such dharmic virtues as humility, patience, compassion, chastity, which are the subject of Vedic ordinances, are illustrated through the noble examples of men belonging to ancient times, women of hallowed reputation. By reading their stories or listening to them we form a deep attachment to the virtues and qualities exemplified by them.
All these men and women whose accounts are contained in the Puranas had to undergo trials and tribulations. We keep commiting so many wrongs. But consider these Puranic characters who had to suffer more than we suffer. Indeed some of them had to go through terrible ordeals. However, by reading their stories we do not form the impression that adherence to dharma means suffering. On the contrary, etched in our minds is the example of men and women of great inner purity who in their practice of dharma stood like a rock against all difficulties and challenges. At the same time, we moved by their tales of woe and thereby our own inner impurities are washed away. Finally, the glorious victory they achieve in the end and fame they achieve help to create a sturdy bond in us with dharma.
For a general background, please see here