Periscopes on heavy armored vehicles.
Now them have only 1 periscope with very narrow FOV, what makes very hard to be a driver without 3D view (most realistic settings, what I like).
In real life most of them have 3 or more periscopes.
Changing current periscope can be realized through pressing [look right] and [look left] buttons (by default - Num4 and Num6).
Another way - enable camera turning and replace "one periscope" texture mask to "multiple periscopes". Not sure that it possible.