Yudhisthira is often called dharma-raja. His conduct throughout the epic as well as his conversation with the Yaksha shows that he knows all about dharma, at least theoretically. And yet, their conversation is not a dry academic exercise between two people who know all the answers. As a practical matter, the new king needs to know "how does one rule in dharma?"
Apart from his personal interest in enlightenment and truth, Yudhisthira is interested in how to maintain the balance of the cosmic order. This question forms the basis of their dialogue. Bhishma points out that spiritual truth may not always form the basis of political action. Maintaining a balance is no easy task. He often speaks as an oracle whose truths are ambiguous and deep. While this world is temporary and we must strive for spiritual perfection, Bhishma advises Yudhisthira to be a king and play his role within the dharmic system until he is no longer necessary.
Maintaining the tension between social dharma and spiritual realization is one of the key difficulties throughout the Mahabharata and Bhishma's teachings. And so the old warrior, while waiting for his own spiritual destiny to unfold at the moment of his death, weaves the fabric of raja-dharma with his words. Telling fascinating stories that avoid touching on the main narrative of Mahabharata, Bhishma speaks at great length on how kings should think and act.