Why does stronger magnets improve a dc motor?
Stronger magnets change the characteristics of the motor.  This may improve the motor or it may not depending on what you want the motor to do.

Torque develops in an electric motor from the magnetic fields of the rotor attempting to align with the magnetic fields of the stater.  The stronger the magnetic field the stronger the force.  So a stronger magnet allows for greater torque.

The trade off is max speed at a given voltage.  This is because a stronger permanent magnet means more magnetic field lines cutting across the copper windings increasing the back emf for a given speed..