Albert Barnes Commentary Isaiah 21:6

"For thus hath the Lord said unto me, Go, set a watchman: let him declare what he seeth:" — Isaiah 21:6 (ASV)

Go, set a watchman - This was said to Isaiah in the vision. He represents himself as in Babylon, and as hearing God command him to set a watchman on the watchtower who would announce what was to happen. All this is designed merely to bring the manner of the destruction of the city more vividly before the eye.