Quite frankly, that's none of your business - using the same logic, why do you need a backup of a cassette or a book or a magazine or a videotape?
The "why" is immaterial.
You want to debate the logic of actually getting it, fine. But when you get into challenging the "why", you begin treading on an individuals sovreign rights. For all you know, the backup copy is so that the original can be crushed in a compactor. As long as the copy is not being sold, it shouldn't matter.
Anyone else see the irony in Kazaa complaining about copyright infringement?