Take it from an Internal Medicine physician (retired now; but what I used to tell my patients) using liners for a full face mask x 8 years: 1): acts like a gasket to prevent leaks: every interface needs a gasket (think plumbing, car engine...); every fitting has a gasket. Gaskets prevent leaks. Same thing with the mask. 2. Slumber: when winding down to go to sleep, nothing wakes one up quicker than putting cold plastic/rubber on one's face and "suddenly, I"m awake". Startled. The liner is like having a high-thread-count pillow case on the mask making it warm and comfy; just like first laying down your head swaddled into a comfy pillow. Love them.