I’ve booked plenty of airline tickets never used them. Not once
It’s transparently being used behind the scenes. Over 225 airlines around the world use it. A long time ago, pre-internet, I think there was an interface to Sabre via CompuServe that people could use it directly.
If you booked a flight in tye USA then the airline data would most likely have been funnelled through Sabre’s systems. And the online agency would have used Sabre’s aggregation , offer, order and post booking systems