The word zu is an archaic or poetic form of the relative pronoun and demonstrative particle, equivalent to zeh (H2088, 'this') or the relative particle asher (H834, 'which/that'). It appears primarily in ancient poetry, the Psalms, and prophetic literature, lending a majestic, elevated tone to the text. Its brevity and archaism give it a solemn, almost hymnic quality.
Although a grammatical particle rather than a content word, zu appears in some of Scripture's most majestic hymnic passages. In Psalm 68:8, it is used in the great victory hymn of God's march through the wilderness. Its archaic flavor evokes the ancient covenant songs — Moses' Song of the Sea (Exodus 15) and the songs of the judges. The very grammar of Israel's worship poetry reflects the antiquity and majesty of their God, whose ways are ancient and whose acts are celebrated across generations.