Freewheeling pulley, with a SPRAG clutch, allows the alternator to freewheel and bleed off some inertia between engine torque pulses and abrupt higher RPM engine speed changes.
Every time a cylinder is compressing, the crankshaft slows down. Every time a cylinder fires, the crankshaft accelerates, even at a "constant" RPM. Especially when there is high load.
Since drive belts can only "pull", the belt also speeds up and slows down which creates vibration and whipping. Since the alternator has the smallest pulley, it sees the most from these accelerations and provides the most rotational inertia. With the Sprag clutch, when the engine slows down, it prevents making the other (normally tensioned) side slack!
Esprit engines do not have a spring loaded idler pulley, like most of them.
Advantages are spelled out here:
One needs to find the largest OD pulley available to be able to machine a V-belt groove into it. Mine measured 71mm OD and was advertised as for Corvette application. Spent hours on eBay looking for a suitable donor. The one I have used did not have any part number (bought the whole alternator and separated the clutch pulley). I found similar part, I think.(?)
Part Brand: LITENS OAD
Other Part Number: TENDECO 920746
Manufacturer Part Number: 39329
Interchange Part Number: VALEO 2601628A, 2601628B