A sample program implementing a headless Spotify Connect receiver. You can use this app to play audio from Spotify on your system using a remote client(like spotify for your phone, spotify-qt app for desktop etc.) that can send requests to spotify API. Not just this, you can do many more things just run
librespot --help to list usage.
librespot -u USERNAME -p PASSWORD -n librespot_server
The above command will create a device names librespot_server which will play audio on your system when requested by remotes.
librespot -n "Librespot" -b 320 -c ./cache --enable-volume-normalisation --initial-volume 75 --device-type avr
The above command will create a receiver named Librespot, with bitrate set to 320kbps, initial volume at 75%, with volume normalisation enabled, and the device displayed in the app as an Audio/Video Receiver. A folder named cache will be created/used in the current directory, and be used to cache audio data and credentials.