It was originally used by fish as a hard protective outer case, similar to the shells of molluscs and the outer skeleton of arthropods, and it is still used like this around the brain.
But of all the major groups of animals, only in vertebrates has the hard covering evolved into a hard inner framework, surrounded by the softer parts of the body. This makes vertebrates vulnerable to damage, but it also gives them the advantage of being able to put on weight and loose weight without having to reshape their outer shell.