Apparently, it's not to hear what they say they know. It's to get them to say what you want to hear. McClatchy's D.C. web site reports that the info the Bush administration wanted from their suspects was the link between Al-Qaida and Saddam, a link which would justify the Iraq war:
"There were two reasons why these interrogations were so persistent, and why extreme methods were used," the former senior intelligence official said on condition of anonymity because of the issue's sensitivity.
"The main one is that everyone was worried about some kind of follow-up attack (after 9/11). But for most of 2002 and into 2003, Cheney and Rumsfeld, especially, were also demanding proof of the links between al Qaida and Iraq that (former Iraqi exile leader Ahmed) Chalabi and others had told them were there." . . . "There was constant pressure on the intelligence agencies and the interrogators to do whatever it took to get that information out of the detainees, especially the few high-value ones we had, and when people kept coming up empty, they were told by Cheney's and Rumsfeld's people to push harder," he continued.
Read the whole McClatchy story here.