You have to bind a key to its use because by default there is none.Same goes for binocular view.Binocular view is the most important one.
Defiantly bind the Binocular View. Its a little awkward at first but worth getting used to.
Advantages:
Magnified view from the commanders position so you get a better look over hills etc than you do in the gun sight.
Rotates faster than turret so you can scan faster when needed.
When in bino view hitting the left mouse button orders the gunner to traverse to your view. Right Mouse button takes you to the gun.
So your scanning with the binos and id a target, hit left mouse, wait for the gun to lay on, hit right mouse, adjust for range and fire.