In a perfect world, all music recorded over the last fifty years would be available on multi-track tape formulated to last for centuries. Unfortunately, the world is not perfect.
A Classic Mix Is too Sacred to Touch
There Is Not Enough Time or Budget to do a New Mix
A Multitrack Source Tape in a Bad Condition
Sound Effects Have to be Spread to 5.1 for Cinema Presentation or HD Broadcast
An Up-converted Version can be used as a Benchmark when Remixing