oxygen is a very hot gas so if humidification isnt used, the oxygen will burn the respiratory tract and cause considerable pain for the patient when they breathe

Oxygen is a dry gas which can cause evaporation of water from the respiratory tract and lead to thickened mucus in the airways, reduction of the movement of cilia and increased susceptibility to respiratory infection.

Humidification cleans the oxygen as it is administered to ensure it is free from any aerobic pathogens before it is inhaled by the patient.