Controlling the meeting calendar for the president is more than just the clerical work of a personal assistant. The president works and makes decisions by meeting with people. She decides who gets access to him today. She deflects people she doesn’t think are worth his time. Over the long run she can have extraordinary influence over him and what choices he has in front of him to make. She can effectively block people she doesn’t want to have any access. Thats a lot of power for someone most people don’t know by name.