Obama understand more than most that part of making a policy/presidency successful, is knowing how to market it. Lawrence Summers is by all accounts an economic genius. He also lacks social grace and some people such as author of Richard Bradley who wrote Harvard Rules  even found some people to go as far as to speculate that he is borderline autistic. (something  I think is mostly silly) However, because of his lack of social skills and knack for saying things that piss a lot of people off (such as his women and math comment) , he would not be a good pick to represent the Obama administration economic policy. Enter in Timothy Geithner as Secretary of the Treasury. Geithner, not only has credentials for the job, but he is younger, a fresher face, and much better at speaking in public than Summers. However, Obama is smart enough to know he needs a mind like Summers as well so he named Summers as director of the National Economic Council where he can craft policy but not be as much in the spotlight where he has the potential to do or say odd things.