When I fly with an oxygen installation (Mountain High EDS is very popular in gliders) I have it active from 5,000’PA. Some of the airfields I fly from are near/above that and you want all your wits about you when you are close to the ground. I’ve done flights with a pulse oximeter and it’s amazing what a difference just a little bit of supplemental oxygen has, if the delivery system is any good.
Close to the rock in the mountains and in extreme conditions in places like Namibia and South Africa, it’s a real safety thing.