A crossbow bolt spins for the same reason that a football or golf ball does because it is traveling through the air at high speed and is subject to the laws of aerodynamics. The difference is that a crossbow bolt spinning at high speed creates a lot of gyroscopic force, which acts to stabilize the bolt in flight and keep it pointed in the same direction. This is why it is so important to ensure that your crossbow bolts are properly spined for your particular bow; if they are not, the bolts will fly erratically and be difficult to hit your target.