Because otherwise it can get quite claustrophobic.

Our vision, which works so well in air, all of a sudden is very restricted and claustrophobic.

Romeijn said the engineer's room was 'kind of claustrophobic and the noise of the engine must have been tremendous'

I remember feeling claustrophobic as I walked down its narrow halls and into classrooms turned into crude cellblocks. The air was stale but heavy with the stench of death in interrogation chambers, barren save for a single bed frame, shackles, and a chair.

You start to realize that Hey, I could go to this live music event and have an utterly transforming experience that will cover my entire body with goosebumps, but it's more likely that I'll feel claustrophobic and I won't be able to get a beer.