I think this can hinge on what you think about Saul "calling up" Samuel. If one believes in that case Samuel was instead a demonic apparition, then this means that demons can and do mimic "spiritual/occult" things as part of their stratagem for evil enterprise. In the case of other religions, one of the obvious reasons would be to give a sense of legitimacy to that religion and its teachers, thus keeping its adherents blind to the truth, and making the skeptic possibly believe.