If you have a public url for a file, then you can just point your users to that url and use sendfile=on. Nginx will then use sendfile. But consider the usecase

If you have a public url for a file, then you can just point your users to that url and use sendfile=on. Nginx will then use sendfile. But consider the usecase where you have a bigger download hidden behind some kind of authentication. You'd do that authentication in your normal backend and return the X-Accel-Redirect header in your response. Nginx will then take over and send the data using sendfile.