In the progress bar of the now playing screen and mini player there is a bar that moves along with the playback position. I assume this shows the cached part of the song if you stream a song, but for locally stored music this seems to be unnecessary.
None, it would just be nicer in my opinion.
The user would be able to discern whether the played song is stored locally/cached or is currently streaming.
Other players, that play locally stored music don’t display this bar.