This wearable item occupies a specific slot for rebreathers, which are of two kinds.
The ordinary looted, wearable rebreather lets you pass through certain places while taking no damage, such as the Geonosian Caves.
Using an Alum Mine Gel Packet which can be looted in the Death Watch Bunker on Endor, a trader can create an Advanced Rebreather (the modified item will hold the same name though) which stops damage being inflicted via gas in the mines under the bunker.
It is definitely useful to proceed further down the bunker.
Wearing the Mandalorian armor helmet will filter all the known gases, due to its built-in rebreather.