Explanation
OAuth is the preferred means to authenticate because it provides access to both the OData RESTful web services (Web API and OData global Discovery service) as well as to the SOAP web services (Organization service and Discovery service).
OAuth is also required to support:
Azure Active Directory configurations for conditional access, such as Two-factor Authentication (2FA) Use of client secrets to enable server-to-server authentication scenarios.
Cross-Origin Resource Sharing (CORS) to connect a Single-page Application (SPA) Reference:
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/authentication