If a fire breaks out onsatge (and remember, theatres used to use live flame lamps) the safety curtain falls into place (it can be tripped or it will be tripped when the heat causes the thermostatic release to trigger) and keeps the fire from entering the audience long enough for the patrons to flee the theatre (and, hopefully not be killed in a stampede.) The curtains used to be made of one-half-inch thick asbestos, which probably explained why so many actors used to die of lung cancer.
Do you know how they used to figure out the maximim number of seats in a theatre? They would make a scale model of the theatre house with seating in place and the exit door open. Then they would pour steel ball-bearings into the model - one for each patron - and shake the model. All the ball bearings had to clear the theatre in 120 seconds (and they could be added incrementally until the limit was reached). It was assumed this would measure the capacity seating.
http://www.broadwayworld.com/board/readmessage.cfm?thread=972787#3631451
http://www.broadwayworld.com/board/readmessage.cfm?thread=963561#3533883
http://www.broadwayworld.com/board/readmessage.cfm?thread=955158#3440952
http://www.broadwayworld.com/board/readmessage.cfm?thread=954269#3427915
http://www.broadwayworld.com/board/readmessage.cfm?thread=955012#3441622
http://www.broadwayworld.com/board/readmessage.cfm?thread=954344#3428699