What is the purpose of a rotor in an alternator?

What is the purpose of a rotor in an alternator?
It's the bit that spins in the middle, turned by the the drive belt.
The rotor contains coils of wire, which are connected via slip rings to the control circuitry.
Dc power applied to the rotor, turns it into an electromagnet. This will then induce an AC current in the stator windings, as the rotor turns within it.
The strength of the 'rotor' electromagnet, can be varied by the control circuitry, to maintain the voltage and control the current demand.
The AC outputs from the stator coils are rectifed (ususally three phase) back into DC, for use in the cars electrical system and battery charging.