As mentioned above, an ability to peak from any hidden position without exposing whole upper body.
There is only one type of peaking where you in some situations expose too much of your body, like in small bushes, a short corner with open wall without enough space.
Like peaking just with your head/eyes, without your weapon line of fire, and weapon around the corner without sight or with a mirror.
Just to expose your arms far out the corner for higher field of view with a mirror is much appreciated. This mirror just have to work for the client using it.

Regards