Binoculars (or binos) are used primarily to accurately see objects from a long distance. They are carried by NCOs and observers in the Scout squads. While they can be used as a melee weapon, they are a weapon of last resort.
Binoculars can be useful to NCOs in calling down artillery on enemy targets, using those binoculars to sight those targets. However, they are not necessary to call artillery, which can be done using Q-right click with any active weapon. As the binoculars are just as effective in cover as they are standing on top of a trench or in plain view, it is not recommended to sight targets whilst over the parapet.
As a weapon, it is generally treated to be a joke and used by NCOs who have taken their enemy unawares primarily in that manner, though its use is rare, given that an enemy can turn around and shoot in one shot, whilst the binoculars take at least three blows.