Are you answering the question [to the Army ,what would be the purpose of back up tapes?]?
I thought the answer was implicit in my question, but in any event: you make back-ups of everything "just in case." It's more time and money wasted to figure out what to back up and what not to back up than just to back up the whole fucking mess, and if you need it, you've got it and if you don't need it, so what? Besides, there seems to be some unwritten principle of office life, known to me from bitter experience, that just as soon as you decide you don't need a specific file or document, you find out just after it was shredded that you do need it after all. Backing up is insurance against human error in deciding what should or shouldn't be backed up.
<<No there are not always duplicates of ugly secrets.>>
Since the Army was resisting Reuters' courtroom attempts to get the whole tape, right up to and past the time when WikiLeaks got what it showed, I think it's a safe bet that the Army has the whole thing all locked up safe and sound in multiple copies.