Why was Zhu Di made prince of Yan before becoming the Yongle emperor?

Zhu Di was designated the prince of Yan in 1370 as part of the early Ming dynasty governmental system. Under this system, imperial princes—with the exception of the eldest son, who remained in Nanjing as the heir apparent—were enfeoffed in strategic regions to serve as regional viceroys.1

By appointing Zhu Di to this position, the Hongwu emperor placed him in a critical area, as Yan was an ancient name for the region surrounding Beijing. This role allowed Zhu Di to gain significant experience in military leadership, patrolling, and defending the northern frontier against the Mongols. Over time, he assumed supervisory control over the defense forces of the central sector of the northern frontier, which helped cultivate the leadership qualities and military strength that eventually enabled him to seize the throne and become the Yongle emperor.1