The Unggoy's homeworld, Balaho, has a Methane-rich atmosphere and as such Unggoy breathe methane gas. Thus, when they are in a Oxygen-rich atmosphere, like that of the Halo installations or Earth, they need to wear a methane gas tank. When Unggoy are in battle, they may run low on their supply of methane, so they must have a ready supply nearby if required. These tanks contain the ready supply of Methane Gas the Unggoy need.