Why do sawsharks have saws?

They use it in several ways:

• Detecting prey: The saw is covered with tiny sensory organs called ampullae of Lorenzini that detect the weak electrical fields produced by fish, shrimp, and other animals hidden in the sand.
• Digging: They sweep the saw through the seafloor to uncover buried prey.
• Attacking: When they locate prey, they slash the saw rapidly from side to side. The sharp teeth along the edges can stun, injure, or kill small fish before the shark eats them.
• Defense: If threatened, they can also use the saw to fend off predators.