Volume Control
There are just a few proposed standards for encoding to gain and MP3 file, listeners may want to hear the music file like an original soundtrack, but this does not conclude that they want to use volume control for each track being played, but notice the music played on an original compact disc, the volume was intentionally set by a professional mastering engineer, for each track has equal volume, compact discs & other different sources are digitally mastered and recorded at different volumes, so when playing tracks one after the another, it may be convienient to set volume information on the tag being used so when you play it again, the volume dynamically adjusts itself.