Unlike a video, audios and music cannot be seen (although technically you can, if you use a music visualizer). There are times when you need to remove vocals from a song for example, when you cannot find a suitable original accompany but want to make a Karaoke version of a song when you need to split one or more audio tracks/parts for re-mix, mashup or further editing when you want to isolate some instruments like bass, drums and piano to create minus one or a cappella.
