The review mirror serves several purposes. We can determine if an emergency vehicle is behind us and we need to move over. We can see if someone is coming up quickly and we need to adjust our course. We may even see law enforcement coming to correct our negligence in observing the speed laws.
Figuratively, the rearview mirror allows us to look over the last day, week, month, or year, and reflect. We can determine if goals were met, and make adjustments. We can examine what has worked, what has not, and make course corrections.
A look in the rearview mirror can be beneficial.