Brevity is the forte of Vedas. They convey the deepest of meanings and highest of truths in the shortest form possible. The level of Vedic understanding that we humans display depends on the level of our connectivity with the cosmos and grades of our intuition towards nature. Such aspects of our existence varies with time. We have displayed better cosmic connectivity and intuition in the earlier yugas than now in Kaliyuga. Hence the Itihasa Puranas become even more important to us in Kaliyuga than in the previous yugas to understand the essence of the Vedas.
Kanchi Mahaperiyava explains thus.
Take for example the hymn – Sathyam Vadha – Speak the truth – the Taittiriya Upanishad stops with that statement and does not expand on it. But in Markandeya Purana we find King Harischandra’s life and a detailed account of his travails for being someone who always spoke the truth. The meaning of this short hymn and the effect of speaking and not speaking the truth has been chronicled and elaborated in the king’s life history. Dharmam Chara – follow dharma – is another popular hymn whose inner meaning has been exhaustively shown in Mahabaratha through the life of Yudhishtir. Rama showed us the meanings of Maathru devo bhava and Pithru devo Bhava by living the meaning himself. While these are just examples to drive home the point, we will be able to find many instances in all their lives to showcase all the Vedic principles.
The Vedas don’t expand on the meanings of whatever they preach and the Puranas don’t mention the Vedic hymns explicitly while narrating the stories. This is where the śreṣṭha like the pauranikas play their parts. They make both the ends meet and bring out the Vedic message when narrating the Puranic stories thus expanding the reach and coverage of the Vedas to include the masses so that we can appreciate the highest good and build up our inner spirit. Kanchi Mahaperiyava says that while we get to know about their lives and sufferings through Itihasa Puranas, our hearts melt for them thus cleaning away the dirt that we have accumulated in our minds due to our Karma, thereby clearing the way for our evolution-as a Jivatma-to higher levels of existence.