Slaying demons? What demons?

[DragonEmperor23]

Yeah, just saying it's a possibility as something he can do.

He uses it when he's too tired from using Dance of the Fire God to maintain his strength and fight for longer.

 

[Peter1129]

Yeah it's pretty much a move that he uses when he no longer has the stamina to use Dance of the Fire God by itself. Basically this is how Post-Rehabilitation Tanjiro fights.

Tanjiro starts off with Breath of Water. If the enemy blocks everything or is overpowering him he switches to Dance of the Fire God. If he no longer has the stamina to keep up Dance of the Fire God he switches back to Breath of Water and occasionally uses a mix of them both when the situation gets desperate.