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.