Specifically, it matters because on Aug. 18th, 2003, Donald Rumsfeld ordered Army Maj. Gen. Geoffrey D. Miller to conduct an inspection at Abu Ghraib, after Army officers in the field complained of getting no useful intelligence there. The result of these investigations led directly to an Oct. 12 memorandum by Gen. Sanchez, which gave military intelligence a greater degree of control over at Abu Ghraib, which essentially told them to take the gloves off, allowing interrogators to commit acts designed to "manipulate an internee's emotions and weaknesses."
Testimony by Col. Thomas Pappas in the Taguba report details the pressure he felt from higher levels in the Pentagon to extract more information from prisoners at Abu Ghraib. In his testimony, Pappas believed that "policies and procedures" at Abu Ghraib "were enacted as a specific result" of recommendations made by Maj. Gen. Geoffrey Miller, the former commander at Guantanamo.
The question which should be asked of Donald Rumsfeld and General Sanchez -- and soon, in public, and not in some closed Congressional meeting -- is whether General Sanchez ever recieved any orders from Donald Rumsfeld that either specifically stated or implied that more forceful techniques or a change of command over the interrogations at Abu Ghraib should be enacted at Abu Ghraib. If such a thing happened, then Donald Rumsfeld is, at minimum, indirectly responsible for the attrocities at Abu Ghraib and should step down immediately.