That's a great question! Not all lizard species can re-grow their tails, but many can. Lizards have evolved this trait as a way to defend themselves against predators - when a bird or other predator tries to grab the lizard, it will disconnect its tail voluntarily (the lizard can control this). Tail dropping includes the separation of skin, muscles, blood supply, nerves, and bone. The flapping tail will be left with the predator while the lizard can escape. But, it is energetically costly for lizards to have to regrow their tails and the new tail is not physiologically equivalent to their original tail (the tail is generally not identical in length, color, or pattern, and will contain a rod of cartilage in it rather than bone). Lizard tails are important to them for balance as well as a defense mechanism, so if a lizard just dropped its tail and could not regrow it, it would be more difficult for it to escape predators in the future and hunt its own prey.