He’s using the words to unite and amplify the mana of those around him. The lyrics help to hone and focus the mana in a way that allows those who participate to gather more and share it with those around them. (Similar to mages who cast spells to use their magic with more precision and intent.)
It reminds me of something similar to pack mentality in humans. Like in the movies when the battle is near and a song starts to play just as the fight commences. It causes the viewer to feel the same adrenaline and excitement of the battle about to happen. Similar to when watching battles of old when a bagpipe sounds or a drum beats. When the soldiers roar out across the battlefield.
Like imagine you’re standing in front of a great enemy, the battlefield decided and the war imminent. An impossible battle.
All of the sudden from behind you hear the beat of a drum, a lone ‘Dun’ the thrums deep and echos far. A second joins, then a third and fourth. Hundreds now sound creating a hum that emulates a heartbeat. That rushes through your blood in a primal, ancient way. and then a voice joins. A cry of defiance. A song of pending victory. Followed by the bothers and sisters around it. A roar of voices twine with the beat. A story of battles past and present.
With it brings morale. With it brings hope. A song of war. A song of life and death and the future after the struggle is over.
I still don't understand the songs and hymns but the fights are so cool