How do fire sprinklers save lives?

Fire sprinkler systems are fed by pressurised water from the
mains or by their own supply. This water is kept from being
released by a plug inside the sprinkler head that will only
be released when the room’s temperature exceeds 68 degrees
Celsius (155 degrees Fahrenheit). At this temperature, the tiny
ampule that seals the plug mechanism will shatter to release a
deluge of water.
In more vulnerable environments, smaller-diameter ampules
are used for a faster response time. Some sprinkler heads have
spring-loaded plates soldered in place over the plug, and are
activated when the solder melts. These are known as closedtype
sprinklers, because once they are opened, they will spray
water until their supply is cut. The on-off sprinkler features a
heat-sensitive pilot valve that stops the discharge of water
once the temperature has dropped.