Actually there are a few different theories. Some say it was in order to search for the emperor of that time's defeated predecessor. Some say it was display of power directed at other countries. However, most everybody thinks that each of the 7 trips had different objectives. For example, the first time was to pacify the small countries in the southern sea; in order to decrease the number of places Japanese pirates were harassing along the Chinese coast. The second time was trade. Also, many times it was in order to create new shipping routes.