When patients have respiratory problems due to low oxygen saturation then you can supplement the O2 of room air by using high flow oxygen at various concentrations above room air using nasal cannulae or masks such as a Hudson mask.
When they are unable to maintain adequate oxygenation levels with a mask, and/or suffering respiratory muscle fatigue, they are then placed on a mechanical ventilator. The cuff on the intubation tube is to stop secretions falling back into the lung causing aspiration pneumonia as usually at this stage the patient is no longer able to protect their own airways.
Some people have tried CPAP devices to supply air under pressure but that has the potential disadvantage of aerosolizing virus into the environment
While that measure could stretch the supply of ventilators and save lives, it has a major drawback. Officials and scientists have known for years that when used with a face mask, such alternative devices can possibly increase the spread of infectious disease by aerosolizing the virus, whether used in the hospital or at home.
Indeed, that very scenario may have contributed to the spread of COVID-19 within a Washington state nursing home that became ground zero in the United States. First responders called to the Life Care Center of Kirkland starting Feb. 24 initially used positive airway pressure machines, often known as CPAPs, to treat residents before it was known the patients were infected with COVID-19.