Full Face Masks or Integrated Diving Masks, IDMs, combine your primary 2nd stage with your mask for a unique way of diving similar to commercial and deep sea divers because of the many benefits they have. An IDM covers your whole face so your face stays dry and warm while you can breathe through both nose and mouth.
While breathing in fresh air flows over the inside of your mask which clears any condensation so you never have to defog the mask. By covering both nose and mouth you can breathe more naturally and reduce your breathing rate while eliminating jaw fatigue as there is no mouthpiece to bite down on.
IDMs are also more secure with a six point strap holding it in position so it's very unlikely to become dislodged if knocked but they still have a quick release if you need to take it off quickly.