Nietzsche said that, the more abstract your goal is, the more people need to be "seduced by the senses" to it. "humanism" is a nice pure concept of valuing humanity and working together and stuff. But spirituality adds the elements of mystery and magic and divine purpose. Even from the atheist perspective, there's value in spiritual forms, consider the satanic church, accomplishing humanist goals like support for abortion, through the form of a "bible" and "religious organization."