When you are playing a track, and want to play that same track from the beginning again while it is playing, you can not achive that with the player controls.
(The play button changed to pause functionality, and 'previous' does what it says ;-)
With cd players that also combine the play/pause button, there is often the behaviour that when you press 'previous' once, it will take you to the beginning of the current track, and when pressed twice it will go to the previous track.
I would like this for MB too.