freewheeling

A stupid question I know, but what is the rationale behind a free-wheeling alternator pulley? I'd have thought that the alternator would always be generating power and therefore would never free-wheel so I'm obviously missing something. Thanks,

I have a 1994 500 SL (90K miles) and at low speeds when you dont have a foot on the accelerator pedal, it makes a feint scraping noise, almost as if the discs and pads are binding but I don't think it is this because as soon as you apply the gas it stops. Also breaking makes no difference to the...