The Lord says in this passage that when the priest says this, then the Lord will bless them. The jussive then is an oracle, not a wish or a prayer. It is a declaration of what the Lord imparts. It is as binding and sure as a patriarchal blessing which once said officially could not be taken back. The priest here is then pronouncing the word of the Lord, declaring to the congregation the outcome of the atonement.

NET Bible Notes for Numbers 6:24-26