From Halopedia, the Halo wiki
They are used for several functions, allowing face-to-face communication over relative short distance, providing holographic duplicate decoys of important personnel, and engaging enemy forces without risking allied personnel.
Sesa 'Refumee, the Sangheili leader of a faction of heretics based on Basis and a gas mine on Threshold, used one of these to taunt a Covenant Special Operations assassination team, and later used another two to assist him in battle as he tried to kill the Arbiter but failed.
They are relatively weak in battle and a direct shot will result in static washing over their form, betraying their holographic nature. By using an unknown technology, these holo-drones can interact with the environment around them and attack and even physically harm their targets. Though they possess rudimentary shields, and are somehow capable of using actual weapons, they do not seem to be dedicated combat equipment. Physically, they resemble inactive plasma grenades able to fly through the air fairly quickly, though slightly larger.
List of appearances