[CarPsyco84]

I think the reason is when the crank is twisting slightly from the torque output of the rods, that resonance that can occur is the crank basically jerking foreword and then back, kinda like a rubber binder, at a certain frequency that vibration will be very dramatic compared to normal (its resonating frequency). The crank pulley being attached to the water pump and alternator has rotating energy, and like Blackrosenova said, the rubber absorbs those vibrations... I cut metal for a living, you realize how harmonics can affect metals, and what slight changes it takes to fix them...

On a side note, not all engines need dampening, just certain cranks will resonate at a frequency that could be made under use.