Additional protection that new technology afforded. Fuse/Circuit breaker needed a line to neutral short tht exceeded rating to trip; certainly lethal. Ground Fault Circuit Interrupter (GFCI) detects any difference between line and neutral, i.e. you just dropped your blow dryer in the bathtub and the GFCI trips at a level below lethal current flow. Now research has shown some fires are caused by mini faults that don't approach the rating of the breaker and won't trip a GFCI device, but can generate enoughheat to ignite building materials, BTW, some of the newer plastic/styrofoam insulations and sheathing material may have a lower ignition temperature, so we have the arc fault circuit interrupter.
There's a lot of controversy over the AFGI's, including several accustations that they don't prevent fires and don't even detect arcing ground faults. Mike Holt has been covering this for a while now, see mikeholt.com.
