For better or worse, these became code after my place was built!
You can get electrical fires for numerous reasons, but a loose connection that arcs can get things hot enough to burn and not trip the circuit breaker. So, these were mandated for bedrooms, where the risk of a fire at night could be more of a problem. This could occur in a wall and be burning nicely before enough smoke made it into the room to trip the also required smoke detector.
These things will likely (if not already) to be mandated for other areas of a house.
From what I've heard (all hearsay - I've never lived in a place with them), the first generation were prone to false tripping. Something like a hair dryer with brush type motor can generate a lot of sparks, for example. The current ones are said to be less prone to this nuisance tripping. The pros probably have some real-time advice about what currently works, and what to stay away from.