The instructor said it would expose claustrophobia and prepare us for diving calmly in low-vis.
Claustrophobia is interesting. In the Navy Scuba class just before mine a submarine qualified candidate, from diesel boats no less, washed out because he got claustrophobic when they bolted him into a deep sea/heavy gear hat with painted/blacked out windows a standard pre-acceptance test. I was fixing a radio nearby (they called the divers intercom a radio because it looked like something the family gathered around in the 1920s). He didnt even get in the water. I couldnt hear what he said, but they sat him down and unscrewed the hat. He was breathing really hard and sweating, but far from out of control.
I am not sure if a sub-qualified candidate getting claustic was a first-time event in Navy Diving history, but damn unusual at the very least. I always wondered if they let him back on the boats after that or if maybe it was a subconscious reason he left.
I remember my first, and thus far only, experience with claustrophobia. I was maybe 6 years old and my brothers rolled me up in an old rug. I started to freak when I heard them laughing. Possibly out of little brother stubbornness, I figured out that I wasnt going to die or give them the satisfaction. The dread immediately melted away, I quit yelling, and waited for them to unroll me hoping all the time they were horribly disappointed. I guess it cured me.