For some applications, like an API-driven backend made to serve other applications, you can consider that the API itself really is your UI. It's how your users

For some applications, like an API-driven backend made to serve other applications, you can consider that the API itself really is your UI. It's how your users interface to your code. Your users just happen to be other developers whose code had it's own UI too, maybe a text or graphical one or maybe another API.