A fire extinguisher is to clear a safe path to the exit if the fire blocks it. never put them by the exit door as I see so many people do (in their garrage)
they can also be used on small fires but only if there is a safe escape route. (This is probably the most common use but not the primary use)
99% of the benefit of putting the extinguisher by the exit door is that it helps draw panicking people to the exit door. Which gives them another saving throw against Heroic Idiocy, at +4 - "Should I grab this totally unfamiliar little fire extinguisher and carry it back toward the big scary fire, to do something I've never even tried doing? Or should I just run out the handy exit door that I'm right next to?"
they can also be used on small fires but only if there is a safe escape route. (This is probably the most common use but not the primary use)