Zechariah 3:2 "Then [the angel of] Jehovah said to Satan..." New World Translation.

 Zechariah 3:2 "Then [the angel of] Jehovah said to Satan...." New World Translation.

Question: Why does the New World Translation 'add' the words, within square brackets, "the angel of" at Zechariah 3:2?

Firstly the New World Translation is not unique in doing so. The Revised English Bible(1989) reads here at Zechariah 3:2 "The angel said to Satan." (Note it does not use square brackets as the New World Translation does) Its footnote reveals why this choice was adopted for it states: "3:2 angel: prob[able] r[ea]d[in]g, so Syriac; Heb[rew] LORD."-additions in square brackets ours.
This is similiar to what we can read in the footnote to this verse in the Reference Edition of the New World Translation(1984): "2* "The angel (messenger) of Jehovah," Sy[riac]; MLXXVg, "Jehovah."

Other translations that support the New World Translation here are:

The Jerusalem Bible: "The angel of Yahweh...." Footnote: "The angel of Yahweh" Syr.; "Yahweh" Hebr.

New American Bible: "And the angel of the Lord....."

The Holy Bible, An American Translation(W.F. Beck): " "The LORD rebuke you," the Angelº of the LORD said..." Footnote: " ºSyriac; Heb "the LORD said."

The Emphasised Bible(J.B. Rotherham): Footnote: "The messenger of Yahweh(ver.1) appears to be here(ver.2) called "Yahweh"......"

The Bible, An American Translation (Powis-Smith and Goodspeed): "So the angel of the LORD said...."

Joyce Baldwin writes: "[v.1]...The scene is the heavenly court room, where the angel of the Lord, called simply 'the Lord' in verse 2, represents God as judge...."- Haggai, Zechariah, Malachi: An Introduction and Commentary, Tyndale Old Testament Commentaries, Inter-Varsity Press, 1978 reprint(1st pub.1972), page 113.

Also, A New Commentary on Holy Scripture states: "[v]2. the Lord. Read the angel of the Lord, as the speech that follows seems to require."-SPCK, London, reprint 1946(1st pub.1928).