308-harmonic-balancer

Each time a cylinder fires the resulting combustion creates a tremendous amount of pressure, which becomes the force applied to the crankshaft. Each combustion is like a hammer blow to the top of the piston. It hits with enough force to cause the crankshaft not only to turn, but to twist as well. This twisting, along with the accompanying rebound (when the crank rebounds back in the opposite direction), is known as torsional vibration. This explains the broken parts that result in the absence of a good damper.

Here is another way to look at it: imagine a tuning fork. Once hit, it will ring at a certain frequency. A crank will act just like that tuning fork. But, harmonics become destructive to a crank as it reaches its natural frequency. And, the longer the crank remains at this critical point the more severe the damage will be. So you can see why a high revving endurance engine needs a good damper.

Note: The purpose of a harmonic balancer is to control harmonic vibration, not to balance an engine. So even if a motor is zero-balanced it will still need a damper.