Two things come to mind: 1) if any arcing occurs, a metal enclosure will shunt to ground and (hopefully) trip a breaker; 2) in the event of an overload, plastic

Two things come to mind: 1) if any arcing occurs, a metal enclosure will shunt to ground and (hopefully) trip a breaker; 2) in the event of an overload, plastic melts which might result in a fire.