I love when people complain about Gitmo. no reason to close it.
three good reasons off the bat:
1. foreign policy--boosting America's image in Europe so that European leaders are not going to be afraid of political backlash in their countries if they commit troops to Afghanistan. It is easier to sell to the European public that you are sending troops to fight evil along with the good, loving Americans--that is how Europeans perceive Americans, and unlike americans, the euros go the polls and vote and thus wield a lot more power this way than americans.
2. Internal politics. Obama has pissed off plenty of liberals as he is being a centrist and to a certain degree right-wing neoliberal on certain economic issues (see appointments, neither Geithner, nor his trade representative, nor Paul Volcker or Lawrence Summerce are buddies of unions and organized labor). Therefore, Obama needs away to acquire political ammunition against the liberals so that they don't complain later. One way is to close Guantanamo, an issue dear to the hearts of liberals. Trust me, I am surrounded by them.
3. By doing something spectacular really fast, really soon, he ensures his sky-high approval rating throughout the next few months and keeps the media focused on him, thus allowing him to set the agenda of Congress. The president, has very few formal powers that are useful for domestic policy, so the way he gets shit done is through persuasion of Congress. And how he does that? By making sure that if congressmen go against him, their image suffers a blow, something they will want to avoid, as they face re-election much faster than him.