Explanation
The HttpCalloutMock interface allows testing HTTP callouts by returning a predefined response in a test context.
By implementing HttpCalloutMock to return responses per RAML specification, the Apex REST API Clients unit tests can confirm that the API requests and responses match the expected format and values.
Calling the Apex REST API Clients in a test context to get the mock response is also necessary to verify the adherence to the RAML specs. Calling the HttpCalloutMock implementation from the Apex REST API Clients or requiring the Apex REST API Clients to implement the HttpCalloutMock are not valid options because the HttpCalloutMock interface is implemented by a separate class that is passed as a parameter to the Test.setMock method2 References: 1: Idempotent REST APIs 2: Testing HTTP Callouts by Implementing the HttpCalloutMock Interface