Respirator packs provide an auxiliary supply of Oxygen for the wearer. They are used by UNSC personnel who are in an irrespirable atmosphere. Every crewman on a Naval vessel is outfitted with a respirator pack.
The crew of the UNSC Commonwealth were ordered to wear respirator packs when Captain Wallace vented atmosphere in the ship to cut off a fire caused by plasma. This was suggested by Doctor Halsey to Captain Wallace, who readily agreed.
Orbital Drop Shock Troopers use respirators in their helmets.