The elastic band holding the mask in place makes it easier to be used passively because unlike a mouthpiece it doesn’t have to be held in place. This is especially helpful for people who may be too weak to hold the mouthpiece or lack the necessarily mobility to do so.
Some people may simply prefer the mask because it allows them to retain the use of their hands throughout the duration of the treatment.
Unlike a mouthpiece, the mask does not require you to breathe in a very specific way. When you use a mouthpiece the correct method of breathing is in through the mouth and out through your nose.
For small children or those who may have disabilities, such as a learning disability, this could be challenging. Not only must you be able to follow these instructions but it requires a certain amount of continual concentration throughout the duration of the treatment.