I think I've found the purpose of the Benghazi coverup, other than trying to salvage the president's reputation on foreign policy and security.
1. 2008 on the campaign trail: Obama promises to close GTMO. As of the end of 2012, it' still open.
2. May 2011: Osama bin Laden killed by US forces.
3. April 2012: Obama unilaterally declares an end to the war on terror. I believe he needed to do this to legally hold trials for the detainees. They're in a gray area -- not POWs, not domestic criminals. Still, if Obama can convince the public that the war is over, trials will feel better for people who consider the detainees to be POWs, because POWs can be held until the end of a war (if the war's officially over, then it's time to try or release them).
4. November 2012: Democratic Senator Feinstein commissions a report on the feasibility of housing GTMO detainees in the US. She calls the transfer of detainees to the continental US a "viable option." Obama is using her to make his case.
Obama is slowly and steadily preparing the public for the closing of GTMO, and a serious terror attack on a US embassy would undermine the plan.