For most, yes, but there are people who use them while awake... for instance, people who suffer from conditions like ALS or COPD.
Even as someone with just vanilla sleep apnea I'll occasionally strap mine on for a while when suffering from a head cold...certainly helps open things up.
Yeah, you're basically using Positive End-Expiratory Pressure(PEEP) to keep the alveoli from sticking together or developing stiffness. The problem it solves is under normal conditions your lung compliance goes down and with it your Tidal Volume. My understanding is that PEEP works for COPD patients by maintaining enough alveolar pressure that at the next inspiration all of the alveoli in the lung are recruited to store gas instead of only some which both increases surface area and also volume of gas. Both of these factors improve blood oxygenation. I think it has the same effect with pneumonia too for the same reasons.
I am not an RT but I worked for a ventilator company for like 4 years.
Even as someone with just vanilla sleep apnea I'll occasionally strap mine on for a while when suffering from a head cold...certainly helps open things up.