Portable oxygen concentrator use might be a new thing in the military and in the special forces specifically. Emergency oxygen use often becomes necessary due to injury, or when climbing into high altitudes to avoid altitude sickness. Having a portable oxygen concentrator on hand for emergencies is a necessity in the military, because anything can happen.
Liquid and gas tanks are widely used for portable emergency oxygen therapy because they are cheaper than portable oxygen concentrators. This is because they are simply tanks refilled with either compressed oxygen, which is still in its gaseous form, and liquid oxygen.
Compressed oxygen is cheaper, but it can be considerably heavier. This is no good when it comes to having to travel light. Liquid oxygen allows more to be put into one tank, and surprisingly, it's lighter to carry.
These tanks can last a while, and they have the advantage of not needing a battery. The problem special forces or anyone in the military might run into, is running out of the oxygen in the tank, and having to haul more tanks to stay mobile. This is not at all realistic.
A portable concentrator is more helpful in many situations where one would need to stay mobile for a longer period of time. Even though batteries deplete, it's much easier to carry extra batteries in a backpack than it is to carry an extra tank.
One portable oxygen concentrator battery weighs around 2 lbs, where a tank will never weigh any less than 5 lbs. One tank on your back might weigh 5 lbs, but then you have to carry another tank that weighs 5 lbs separately, and take up a lot more space.
It would make more sense to carry a 10 lb concentrator and a couple 2 lb batteries that take up much less room. Even though the weight is more with the concentrator and the batteries, it is much more compact.
Tanks also need to be refilled. A portable oxygen concentrator purifies oxygen from the air that it brings in through the intake vent. The air passes through an intake filter, and then through one more, or two more filters before you breathe it in as a much higher percentage of oxygen. Oxygen is purified to between 85 to 95%, depending on the model and the dosage setting.
Oxygen is a life saver because it helps to treat injuries. The more oxygen your lungs can bring in, the more oxygen will be flowing in your blood. Oxygen keeps the brain cells from dying if there is trauma to the lungs, and will help you to heal faster from an injury. That's why oxygen must always be kept on hand.