I was confused, why would an underwater snail that breathes underwater need a snorkel to breathe.
It seems that snails live in water that sometimes don't have much oxygen dissolved in the water.
In that situation they would die. So they have a snorkel to poke up into the air above and they can get all the oxygen they need.
But, these snails were in a fish tank with a permanent air line pumping air into the water 24 hours a day. Maybe they just did it for practice.