If you use a CPAP with a humidifier, it’s clearly advised you use distilled water. However, when you clean one, how are you supposed to clean the tubing. Using distilled water would be both expensive and difficult.
A CPAP humidifier is a distiller. You could use muddy swamp water in it and the vapor that goes into your lungs would still be pristine.
However, that would make the machine prone to bacterial and scale buildup, and hard to clean. Ease of cleaning is the real reason to use only distilled water in CPAP machines.