The rational part of the soul needs to obtain control over its spirited and appetitive portions because if the tripartite soul did not grant control to the rational, the body would be uncontrollable and act upon every single impulse it had without any rational thought. Certain spontaneous thoughts and actions can not cause any harm to the body, but others can; therefore, the rational must always be present. The rational portion of the soul is the overseer or safety net, and rationality will always be the judge of what is good and bad/ right or wrong.
In conclusion, the rational portion of the soul should always remain in control over the appetitive and spirited portions. Although not all spontaneous and sudden decisions are harmful, some are. Having the rational portion as a safety net, allows sorting of the impulses, and ultimately leads to a happier and safer life