But, what's the bag for?

You know, on airline oxygen masks?

“The bag may not inflate”
Fine. Great. Whatever.
But, what’s it for?

Gah! Too slow.

Guess I should have searched.

Someone with more knowledge than I will hopefully be in shortly, but I believe it’s there to hold the oxygen that’s flowing to the mask while you’re exhaling. Oxygen flows to the mask continuously, but you obviously can’t make use of it while you’re breathing out, so the bag acts as a reservoir during that phase of the breathing cycle. How much the bag inflates then depends on your lung capacity, and how much of that reservoir you drain out when you inhale.

I’m repeating a half-remembered factoid, though, and I welcome correction if I’m completely off-base.

ETA: Now I know where I remember that factoid from.