Because in that culture and society he's supposed to go to school. Maybe the imperial family simply wants all noble children to spend a few years in the imperial capital to make them better feel like their belong to the empire. If they rule a planet far away from the capital planet, having never even visited it, they might start to become detached. On the other hand, even the parents of the noble children might want their kids to form some connections with other noble children during the school years.
That is, it might not have much to do with actual education. You could receive that anywhere. Though it could serve the role of having all the noble kids receive a portion of the same education. Otherwise they might all learn history, for example, differently on their home planets. On the imperial capital planet, they would learn the sort of history the imperial family dictated to be correct.