They originated on the 1.9 rotary pump diesels. The torque pulses from the crank, combined with the rotational intertia of the alternator combined to unscrew the crank sprocket bolt
The resulting effect (crank sprocket falling off) destroyed many engines before someone figured out what was happening. In VAG stuff the clutched pulley turned up in the 90hp TDI engine I believe and then became ubiquitous on diesels, spread to petrol engines, and now appears to be on almost everything.
It may be a refinement thing in the D2 V6, as the A4 with the same engine doesn't have it.