By being reminded that the Lord was in his holy temple in Habakkuk 2:20, the prophet turned to him in prayer. As a prophet he served God’s people as a covenantal mediator who offered prayers to God on their behalf. His prayer is described as according to Shigionoth
which probably referred to a musical term. The word indicated that his prayer may have been accompanied by music. Whether the prophet played a musical instrument or heard one being played while he prayed is not mentioned. But the expression means that he was moved to the depths of his being with the conflicting emotions of God showing his mercy to his people while he brought the day of trouble upon the Babylonians (Habakkuk 3:2, Habakkuk 3:16). Martyn Lloyd-Jones described the term as being neither mournful, nor joyful, but as an expression of a profound and strong emotion.
The prophet prayed because he was dependent upon God to explain to him his purpose regarding the Babylonian invasion and their defeat.
1 A prayer of Habakkuk the prophet, according to Shigionoth.