The external docs property for the OpenAPI specification provides an opportunity to ensure the supporting landing page, documentation, or other relevant links travel with the technical contract for each API. It is common for OpenAPI artifacts to be forked, distributed, and published at different stages of the API lifecycle, so baking documentation into it is important.
Providing an external docs property completes the loop with your OpenAPI, which is used to generate API documentation, but should also possess a link to your API documentation. Just makes sure that you link to a place that provides access to other resources beyond just documentation, taking care of whatever consumers may be looking for.