From the nobles' perspective, the Forbidden Region holds no value, with most considering it nothing but a tradition or formality to guard the forested place. However the older Nobles (those older than 800 years) know that it's the home of Rai.
Rai is the landlord of the forbidden region. He used to live in a Mansion situated inside the region. Later it was revealed that the area was made forbidden because The Previous Lord left half of Ragnarok at Rai's mansion.