Well Daddy I'm ready for my second one... this is necessary.
Use of mirrors is every 5-8 seconds. No matter what you are doing, your eyes need to return toward the windshield and look forward ahead of you every 2 seconds. That way you are never fixating on one mirror or gauge for more than 2 secs, which is called scan, don't stare.
It's also helpful when changing lanes, that way you are not distracted from the vehicles ahead while checking your lanes and merging. You keep looking in front of you every 2 seconds.
This prevents you from rear ending someone.