I've always been a huge proponent of watching your mirrors at all times. I think when it comes to safety, watching your mirrors should be one of your very top priorities - just a smudge under the importance of watching out your windshield. If you know what is going on around you at all times you'll almost always see this kind of stuff coming (about the one minute mark):
While I was waiting for pedestrians to get across the sidewalk a car drove around me to make the right hand turn in front of me and almost caused a collision.
You want keep glancing in your mirrors and noting the speed that everyone around you is running. Knowing the speeds of all vehicles on all sides of you should give you a picture in your mind of what's going to be happening in the next few seconds. Who is going to switch lanes? Who is going to slow down or speed up? Who's getting on or off the ramps? Will someone be in the lane next to me or will it be clear? Is there a shoulder or (Heaven forbid) a field or a median I can use as an escape if something goes wrong in front of me?
Envisioning the speeds of all vehicles around you makes it pretty easy to predict what most people around you will do in the next 5-10 seconds. That helps you plan your "outs" or your "escape route" in case something drastic happens around you.
And remember, bad things don't just happen in front of you. I've had plenty of vehicles go sideways and start spinning alongside me over the years and a couple of times I swerved over enough to avoid contact. If I hadn't been watching my mirrors I would have been involved in a wreck that was happening 50 feet behind the cab that I had nothing to do with at all.
The situation is the video is the perfect example of how glancing in your left hand mirror during the process of a right hand turn can help you avoid a wreck. Now who would've thought you should be watching the opposite mirror in a turn? Who would have thought someone was about to make a crazy move like that?
Well, if you saw him coming up alongside you that almost certainly would have thrown a red flag in your mind. In the process of watching your mirrors a minute or two earlier you may have noticed that he was driving aggressively and looking to pass people. Maybe he was tailgating you for a little while and you could see he was annoyed. So when you stopped to make a turn and you saw him suddenly come up quickly alongside you, you would have thought, "Oh geez. Here comes this guy. He's probably fed up with me and he's going to do something dumb." Then sure enough, right on cue like you're a fortune teller, he makes some crazy move like that and you sit back safely and watch, shrug your shoulders (cuz it happens all the time out there), and calmly continue on safely with your trip.
Just another hour in another day for truckers.
Remember, things happen in a flash out there. When you're driving an 80,000 pound building on wheels there is no room for mistakes. There may be no second chances. Watching those mirrors will help you predict what others will do and help you determine where your safe zones will be in an emergency. If you're not checking your mirrors regularly you're far more likely to get in a wreck.