The last thing you want while driving 70 mph down the freeway is to be blindsided by another car because you didn't see him in your blind spot.
I've written about this before. Driving down the freeway is difficult enough without having your mirrors set all wrong. You want to be able to see all around you, including your blind spots, and your mirrors can help you do that.
You can read here how to rightly set your rear view mirrors so that blind spots are virtually eliminated.
Using your mirrors wisely will make it far easier to negotiate freeways and interstate roads. You'll be safer and so will the driver coming up beside you on either side.
So if you don't already have your mirrors set correctly, please do take the time to read how to do this. It takes a few miles of practice to feel at ease with a new mirror setup but take it from me, this works wonders for your self-confidence on the freeway.