What is the benefit to using the ARB rear-facing?

The anti-rebound bar improves the stability of the child seat when installed in a rear-facing position and controls rebound in a collision (rebound is movement after the initial collision when the car seat bounces up and back toward the vehicle seatback).

The anti-rebound bar can also help give you a tight Rear-Facing install since it provides a large surface area that typically nests into the flat portion of the vehicle seat back.