Plato gives the Demiurge a motive that is both simple and radical: he was good, and so he wanted everything else to be good too. This is not the arbitrary will of a powerful deity, nor the creative overflow of a self-sufficient being. It is the natural generosity of excellence. Because the Demiurge possessed goodness, he could not withhold it — he desired that all things should resemble himself as far as possible.
Before the Demiurge could act, he needed a model. He looked, Plato says, to the unchangeable rather than the created — to the eternal Forms rather than to any existing thing. The world he made is therefore a copy of a perfect original: beautiful insofar as it resembles its pattern, imperfect insofar as matter resists the imprint of form. This explains why the cosmos is orderly and not chaotic, but also why it falls short of the eternal. It is the best that can be made of what can be made.
The Demiurge's first act was to impose intelligence on soul and soul on body. Finding the visible realm moving in disorder, he brought order — not by force but by reason working through the world-soul he created. The living cosmos that resulted is intelligent, ensouled, and spherical: the fairest shape, containing all others, rotating on its own axis, requiring nothing outside itself.
Plato is careful to mark what the Demiurge is not. He is not omnipotent: he works within the constraints of Necessity, the recalcitrant element that resists his designs. He is not eternal in the way the Forms are eternal: he is a cause, not a principle. And he does not act alone: having fashioned the world-soul and the celestial gods, he delegates the creation of mortal creatures to them, keeping his own hands clean of what is perishable and imperfect. The Demiurge is the best cause in the best possible world — which is not the same as a perfect cause in a perfect world.
The Demiurge appears throughout Chapter 2 of the Timaeus. The concept influenced Neoplatonism profoundly: Plotinus treated the Demiurge as Nous (Intellect), the second hypostasis after the One. In Gnostic thought, the Demiurge was often recast as an inferior or malevolent creator — an inversion Plato would not have recognised.