Most people believe their home or business will never catch fire. If their property does have a fire though, fire alarms are the first line of defense. Not only will they detect smoke emanating from the source, but fire alarms will warn people inside and nearby that there is immediate danger.
There are a couple of reasons why this is so important. First, if residents are sleeping, they will be given warning to safely evacuate the premises or to address the fire, if possible. Second, if there is no one there, an alarm system will alert authorities to come put out the fire and salvage the building, which would otherwise not be possible.