Along with bestowing good juju upon your bike, many also attempt to ward off bad juju with guardian bells. The legend goes that evil spirits try to latch onto your bike and cause mischief, mechanical failures, and crashes. A bell on your bike will attract these spirits, then trap them inside. The constant jangling of the bell drives them insane, at which point they let go and fall harmlessly to the ground.
Another important aspect of this ritual is that you may not buy your own guardian bell. You must receive it as a gift for it to be effective. This helps build friendship and camaraderie among riders. The only disadvantage I can see is if you ride a relatively quiet bike, and the incessant jangling of the bell drives you insane instead of the spirits.