This only happens in an emergency, not during normal operation. Pretty much the only time you would do it when a person is stuck in the machine by some piece o

This only happens in an emergency, not during normal operation. Pretty much the only time you would do it when a person is stuck in the machine by some piece of metal that is attracted to the magnet. You hit the quench button which vents all the helium, but more importantly also halts the magnet (all the energy released from halting the magnet goes into boiling the helium), and then you can release the person. You don't do this for any other purpose, for example if some metal cart is stuck in the machine, but no one is at risk, you use a slower method to shut down the magnet (i.e. drain the energy) that doesn't vent the Helium.