Spotify released a new Connect feature for premium users today, that lets you play Spotify through your computer while using your iOS or Android based phone or tablet as a remote control.
Make sure you have the latest version of Spotify on your PC and phone or tablet and start playing any song. Tap the Now Playing bar, hit the connect icon, pick your computer from the list, and you're all set for controlling your music remotely.
This feature is ideal for a party when you want the music to be louder than what your phone or tablet is capable of, or if you're chilling on your comfy couch and don't want to get up to change songs. If you want to stop PC playback, just tap the connect icon again and select your device.
Connect for Computers [Spotify Blog]