The large spartan office makes a powerful statement - "I can afford all this space, and you come before me as a supplicant" is a pretty strong statement. It takes a pretty big personality to carry it off. The 'I love me' wall humanizes and softens that statement, it also gives a way to make connections with visitors in other than a purely business manner. Social chitchat is at least as important as getting right to the point.  Even the purely formal offices of high government officials have personal mementos scattered around, check out the differences in the Oval Office from President Regan and President Obama, they are there, not so subtle and reflective of the personalities of the officeholder.  Even if the office itself hasn't changed much since being refurbished in the Truman administration.