Not all of us live in Friends size rent stabilized two bedroom lofts. So having a storage unit is a common necessity in Manhattan. The can be a bit creepy, especially when you hear the elevator open two aisles over or when you're walking past an aisle and look down it to see a person's life posessions spread in a semicircle around the open door.
I've read stories about people trying to live in one of these, but it's somewhat hard to imagine locking yourself in like that.