Why did the Yongle emperor create the Eastern Depot in 1420?

The Yongle emperor established the Eastern Depot (Dongchang) in 1420 primarily to serve as a specialized agency tasked with ferreting out treasonable activities.1

As a ruler who was domineering and deeply jealous of his authority, the emperor relied heavily on eunuchs to perform duties outside their traditional palace roles, including acting as regional overseers and supervisors of special projects. By creating this agency, he institutionalized a system of surveillance that allowed him to maintain tighter control over his administration and identify potential threats to his power. While the Eastern Depot did not become notorious during his own reign, it eventually evolved into a feared secret police force that operated in collaboration with the imperial bodyguard in the decades and centuries that followed.1