The Sphinx was believed to have magical powers. So the Sphinx was there magician and he pulled rabbits out of its crown and made things disappear...just kidding. They also believed that the Sphinx was the guardian to the underworld. So the Sphinx was like the judge and the jury choosing whether they go to the underworld or the after life. But why would they trust an inanimate object to be a guardin some a robber could just walk in and steal something and no body will know?