For the sadistic among your gang. Impale the opposition mercilessly. For maximum effect, use a fast vehicle.
The best effect is when you make a glancing blow to any side of your opponent. This way you take less damage while giving the most damage.
Like the Mounted Ram, the spikes will give an extra buffer of armor if not used to skewer your enemy. They also take up a weapon slot when mounted.
Spikes cause extra damage based on your speed.
Generally, smaller, faster cars are more effective with spikes while bigger vehicles are more effective with rams.