How many people did their undergrad at one school and decided that, as much as they loved their time, it would be in their best interest to pursue a grad program at a different school? Even if the original school has a great program sometimes its just best for the student to have a different experience and build new relationships. For a player he would be learning a new system, new style of play, different style of coaching. All of that is invaluable for someone that wants to have a long pro career that would involve lots of different teams and systems.