It’s extremely important that your oxygen concentrator is able to deliver the amount of oxygen you need at any given time. This amount, which is determined by your doctor, will fluctuate depending on how you are breathing. We aren’t always breathing the same way at different points throughout the day. Your breathing will change while you are sleeping, and it will change while you are doing something even slightly physically exerting, like walking from room to room in your house. It definitely changes when you are exercising.
This fluctuation is why a reserve capacity is important in an oxygen concentrator. Your concentrator might be able to sufficiently meet your oxygen therapy needs, and keep you saturated while you are breathing at rest, but without a good reserve capacity, it won’t deliver enough oxygen while you are breathing heavily from walking up a few flights of stairs.