Why did the Hongwu emperor grant principalities to his sons?

The Hongwu emperor granted principalities to his sons primarily as a strategic measure to bolster the defense of the Ming dynasty. Ostensibly, he feared the possibility of another Mongol invasion and sought to provide the imperial princes with military powers to support the regular armies.1

Additionally, the decision was driven by the emperor's desire to maintain personal control over the empire through his sons' principalities. By distributing these territories, he aimed to secure his authority across the realm, though this policy eventually created significant internal friction; after his death, his successor, the Jianwen emperor, attempted to assert control over these powerful uncles, which ultimately provoked a rebellion and led to the usurpation of the throne by the Yongle emperor.123