“The mission has legal and political issues that may lead to interrogators being called to testify, keeping the number of documents with interrogation information to a minimum can minimize certain legal issues,” the document is quoted as saying in an affidavit signed by Kuebler.
For a corporation involved in civil litigation against other corporations, minimizing the amount of stuff available for discovery is a perfectly reasonable, if sometimes sleazy, strategy. For a government all of whose members are pledged to uphold the law, not so much. If they had really thought what they were doing was legitimate, they would have preserved the evidence, not destroyed it. (And if it ever comes to criminal prosecution, this will be important, because a policy of destroying evidence pretty much eliminates the defense of believing that the orders in question were legal.)
This kind of procedure also reinforces the conclusion that interrogators were torturing people just for the sake of torturing them. If you really want information from an interrogation, you preserve every scrap of information from every session, because you never know what little bit might turn out to be important in the context of some other fact that turns up. (There’s an irony — who knows what crucial intelligence from the real terrorists was shredded because of some obstruction-happy lawyer?)