Why is pure oxygen used in respiratory distress?

Acute Respiratory Distress Sindrome (ARDS) is a lung condition caused where the air sacs have too much fluids, this will lower the amount of oxygen, or even increase the amount of carbon dioxide in the lungs, causing breathing failures in such a severe way, that can cause organ failure and even death.

So, doing to its effects the primary goal for doctors when they have a patient with ARDS, it's to ensure that they have enough oxygen inside their lungs, to do so, they provide oxygen by mask, if the condition is too severe, they will proceed to use mechanical ventilation to ensure a certain level of oxygen.

Therefore, the reason why oxygen is needed is because this condition affects lungs, and respiratory mechanism.