Well that depends on what magnets you are talking about. Do you mean the ones in the electrical motor? Or do you mean the ones that are sometimes on the fan blades themselves? If you meant the ones in the electric motor the magnets are there so the motor will work. In a motor you have to have a magnetic field. You also need to have a armature with a field induced in it. The simple principles of magnets come in to play. Opposite poles attract and same poles repel. Therefore the motor turns. If you meant the ones that are sometimes on the fan blades they are there to balance the fan so it doesnt wobble while it is on.