Lots of good examples already, I'll add some too; Turn off air conditioning, turn off furnace, unlock doors, turn lights on, set off various sirens in addition to the smoke alarm, notify you which detector(s) have detected smoke or CO, change driveway light to Red and Flash it for fire department to easily find, if CO alarm turn on whole house fan, change certain inside lights red to alert to get the heck out, triggers cameras to take stills or video so you can verify the issue or just see what's going on, etc etc etc.