All the expeditions were because their country's needed to find new land to conquer. They all were after more land so they could have power over the people that lived there. They also had expeditions to discover new places and to study the world.
People could not buy food or trade goods any more.
The philosophy of respecting the emperor and the Mandate of Heaven was one of the inspirations for the development of filial piety. It is a Confucian philosophy wherein "it is a virtue to respect parents, the elderly, and ancestors." It is of huge importance to the ancient Chinese society because it sets the moral value of people.