C. F. Keil and Franz Delitzsch on Psalms 134:1

Night-Watch Greeting and Counter-Greeting

This Psalm consists of a greeting, Psa 134:1-2, and the reply thereto. The greeting is addressed to those priests and Levites who have the night-watch in the Temple; and this antiphon is purposely placed at the end of the collection of Songs of degrees in order to take the place of a final beracha. In this sense Luther styles this Psalm epiphonema superiorum. It is also in other respects (vid., Symbolae, p. 66) an appropriate finale.