Answer: As the "wind" pushes the blades and makes them to spin, kinda like an inverse process of how a fan works (where in the fan you impunt energy to make the blades move and generate wind, and in this case the wind makes the blades move to generate electricity), and this movement of the blades creates work that is transformed into electric energy.
Without the wind, you can not generate work in the blades, and then electric energy can not be generated, so you need this source to "push" and impulse torque in the blades.