I imagine the slave crown was needed because Terra simply couldn't control her powers. Remember how the Espers that fly out of the Sealed Gate mention that they lost all control of themselves when they left the Esper World and the balancing power of the Statues? Terra even mentions at that point that the same thing happened to her, so it's safe to assume that without the Slave Crown, she would run amok at the slightest provocation, like she did when she came into contact with Tritoch. It's only after she understands her own powers, and no longer fears them that she is able to control them.