While my head is (probably) worth more than the $99 they are asking, it's just a piece of plastic with what looks like an automotive mirror stuck on.
Printable View
It's for ease of applying mascara and lipstick!
Not a chance it's for peeking around corners. It is a bad idea if you think about it. You never stick your weapon into an unchecked space, which you'd have to do to get the mirror far enough to give you a view around a corner, given where it's mounted in the pics.
BUT!!! If you get that picatinny muzzle break, THEN mount this to it... it might work. That'll scream 'bad ass.' [Beer]
It's to keep a eye on your 6. Ya never know when you might become a victim of friendly fire...........
Chances are, if you are the Carl with these things on your weapon, then you are more prone to receiving friendly fire.