According to Dr. Daniel Reynolds, the founder of Lungfish Dive Systems that makes scuba gear, he has suggested that maybe these planes could be used as hyperbaric chambers to treat COVID-19 patients who are experiencing difficulty in breathing (which is one of the more severe symptoms of the disease).
This is because planes usually have to climb to pretty high altitudes during a flight and as a result, the air is too thin for people to breathe. As a result, planes come with the ability to pressurize the cabins to breathable levels, so Reynolds suggests that these grounded aircrafts could increase cabin pressure between 1.6 to 2 atmospheres, essentially transforming them into makeshift hyperbaric chambers.
Reynolds is not alone in floating the idea of using hyperbaric chambers to treat COVID-19 patients suffering from Acute Respiratory Distress Syndrome (ARDS), but for now it is only a theory and further testing and studying needs to be done before it can be used or even approved as a potential treatment.