One primary function of the brake disc or rotor is to

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The brake rotor’s main job is to provide a smooth, durable friction surface for the brake pads to clamp against when you brake. When the pads grip the rotor, their friction converts the vehicle’s kinetic energy into heat, which slows and stops the wheel. Heat management is important and rotors—especially vented ones—help dissipate that heat, but the primary function is to supply the surface for friction. The rotor doesn’t store brake fluid (that’s done by the reservoir/master cylinder) and it doesn’t steer or align the wheel (that’s handled by suspension and steering components).