Spotify gives you a heck of a lot of music right at your fingertips. Whenever you want to listen to something, you can.
Spotify also supports offline storage of music so that you can listen to your favourite music even without a generous mobile dataplan or when you don't have coverage.
Unlike other "modern" mobile os that don't support applications running in the background, Spotify for Android continues to play music even when you're surfing the web or playing games.
Spotify has really revolutionised my way of consuming music.
For more Android app recommendations visit my set: Android Applications.