Both manned and unmanned military vehicles face numerous kinds of direct fire, varying from incoming Small Arms Fire up to high kinetic armor piercing rounds.
Especially when under armor, or unmanned, maintaining situational awareness is key.
The enemy is most likely trying to control the electromagnetic spectrum.
As a consequence, search and track radars will betray the position of the vehicle. They will be jammed when the enemy wants to.
Thus, there is an eminent need for passive sensors on the vehicles.
The combination of optics and acoustics provides interesting sensor fusion synergies.