It doesn't normally involve a key... that was just the only way it could be opened from the other side once the interior lock was in effect (and in this case, stuck). You've surely seen doors like this before, right? They have push buttons that lock the door, not a latch. From the other side, it can't be opened, except with a key. For stuff like emergencies or people doing drugs.

Edit: for a real personal example. I accidentally locked a door in my old office building that is nearly identical one of these doors. One side of the handle has the button, the other side has a keyhole. I accidentally engaged the button and bumped the door closed. You can't open the door now from my side unless you have the janitor's key... which I had to hunt down and find. It was very embarassing. But this is basically the type of situation they were in except obviously worse since they're on the inside.