They are taxes on goods that are imported into a country. A government may use the money from them to fund government operations.
No one knows who wrote the notes
The main reason of why the emperor sent out Zheng He was to increase the Chinese trade and the reputation of his nation.
What important information he brought back was the info on the wind directions, oceans currents, plants and animals he saw, different tribes and kingdoms that would be in for future trade projects.
