The most common title for Haggai in the book is the prophet,
but here he is called the messenger of the Lord
who spoke the Lord’s message.
It is hard to be certain, but it could be that this title is used as part of a wordplay, involving messenger
(מַלְאַ֧ךְ) and message
(בְּמַלְאֲכ֥וּת). Regardless, the use of this title would seem that the emphasis is on Haggai as the representative of the Lord and thus the bearer of a divine message.1
13 Then Haggai, the messenger of the LORD, spoke to the people with the LORD’s message, “I am with you, declares the LORD.”