On a kindly note, you indicate that you would like information as the whys and wherefores involved.
The pulley comprises a driven hub/boss and an outer pulley section, separated by a flexible layer/coupling, which can become detached over time. This flexible component absorbs torsional vibrations, particularly those of high frequency, in order to protect the crankshaft against unwanted stresses.
The pulley therefore comprises a vibration damper, as well as functioning as a drive chain component.
