It's a documentation spec that has a lot of tooling around it. I disagree that it is either pointless or an abstraction.

Back in the day, the best feature of SOAP API's was that they were documented in a machine readable fashion. Point your IDE at the WSDL endpoint and it would autogenerate an API client for you. OpenAPI and the various tooling around it finally offer us something similarly convenient for REST API's.

OpenAPI is a description, not an abstraction.