Even if your frame is metal, the chain can dig deep gouges in the bottom bracket shell/chainstays which is a)unsightly and b) can be a stress point for future failure of the metal...no reason not to have one no matter what your frame is made of.

EDIT: The chaincatcher protects the frame by catching the chain if it is thrown inside the smallest chainring and guiding it back to the ring. Sometimes even a perfectly adjusted drivetrain throws the chain and then it can jam, causing gouges like what the OP experienced. There are different styles of chaincatchers, for both clamp-on and braze-on derailleurs.