Historically, the 使者 was a pivotal figure in Chinese history. In ancient imperial China, emperors would dispatch 使者 to vassal states, rival kingdoms, or distant lands. These were not mere couriers; they were diplomats, spies, explorers, and extensions of imperial power. The success or failure of their mission—and often their very survival—could determine the fate of nations. Famous figures like Zhang Qian (张骞), who was sent to the “Western Regions” during the Han Dynasty, were 使者 whose journeys established the Silk Road. A key difference from the Western concept of a “messenger” is the degree of agency and personal responsibility. A Chinese 使者 often had to interpret the emperor's will from afar, make crucial decisions on the spot, and personify the dignity and authority of their empire. This contrasts with a simple herald who might only recite a pre-written message. The 使者 was the mission in human form, embodying the sender's intent and honor.
While you won't encounter sword-bearing envoys today, the term 使者 remains relevant and is used in several formal and metaphorical contexts.
The word is almost exclusively formal. Using it for everyday tasks, like a food delivery person, would sound comical and out of place.