Why do scuba diver's use a snorkel mask?

Answer 1

During the dive your nose will want to naturally breath in, or take in water so having the nose under the mask helps that NOT to happen. Also if you take on water inside your mask and your nose is not within it will be VERY difficult for you to empty the water.

To empty water from your Scuba mask at depth you simply pull it out from your face and breath out through your nose and this causes the water to pour out. How would you do this if your nose is not enclosed within the mask?

And also so that water doesn't go up your nose. And mouth (it doesn't taste good).

Answer 2

Scuba divers use a snorkel mask to save air from the tank, which is needed when underwater. It is also a safety measure in case air in the tank runs out, in which case the snorkel can be used to return to the boat or shore at the end of the dive.